feat(serve): Opt-in parallelism for `deno serve` (#24920)

Adds a `parallel` flag to `deno serve`. When present, we spawn multiple
workers to parallelize serving requests.


```bash
deno serve --parallel main.ts
```

Currently on linux we use `SO_REUSEPORT` and rely on the fact that the
kernel will distribute connections in a round-robin manner.

On mac and windows, we sort of emulate this by cloning the underlying
file descriptor and passing a handle to each worker. The connections
will not be guaranteed to be fairly distributed (and in practice almost
certainly won't be), but the distribution is still spread enough to
provide a significant performance increase.

---
(Run on an Macbook Pro with an M3 Max, serving `deno.com`

baseline::
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
  2 threads and 125 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   239.78ms   13.56ms 330.54ms   79.12%
    Req/Sec   258.58     35.56   360.00     70.64%
  Latency Distribution
     50%  236.72ms
     75%  248.46ms
     90%  256.84ms
     99%  268.23ms
  15458 requests in 30.02s, 2.47GB read
Requests/sec:    514.89
Transfer/sec:     84.33MB
```

this PR (`with --parallel` flag)
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
  2 threads and 125 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   117.40ms  142.84ms 590.45ms   79.07%
    Req/Sec     1.33k   175.19     1.77k    69.00%
  Latency Distribution
     50%   22.34ms
     75%  223.67ms
     90%  357.32ms
     99%  460.50ms
  79636 requests in 30.07s, 12.74GB read
Requests/sec:   2647.96
Transfer/sec:    433.71MB
```

1 files changed, 58 insertions, 35 deletions

diff --git a/cli/args/flags.rs b/cli/args/flags.rs
index f8577ed1b..800d6ff5a 100644
--- a/cli/args/flags.rs
+++ b/cli/args/flags.rs
@@ -339,6 +339,7 @@ pub struct ServeFlags {
   pub watch: Option<WatchFlagsWithPaths>,
   pub port: u16,
   pub host: String,
+  pub worker_count: Option<usize>,
 }
 
 impl ServeFlags {
@@ -349,6 +350,7 @@ impl ServeFlags {
       watch: None,
       port,
       host: host.to_owned(),
+      worker_count: None,
     }
   }
 }
@@ -2693,6 +2695,9 @@ fn serve_subcommand() -> Command {
         .help("The TCP address to serve on, defaulting to 0.0.0.0 (all interfaces).")
         .value_parser(serve_host_validator),
     )
+    .arg(
+      parallel_arg("multiple server workers", false)
+    )
     .arg(check_arg(false))
     .arg(watch_arg(true))
     .arg(watch_exclude_arg())
@@ -2854,11 +2859,7 @@ Directory arguments are expanded to all contained files matching the glob
           .action(ArgAction::SetTrue),
       )
       .arg(
-        Arg::new("parallel")
-          .long("parallel")
-          .help("Run test modules in parallel. Parallelism defaults to the number of available CPUs or the value in the DENO_JOBS environment variable.")
-          .conflicts_with("jobs")
-          .action(ArgAction::SetTrue)
+        parallel_arg("test modules", true)
       )
       .arg(
         Arg::new("jobs")
@@ -2901,6 +2902,18 @@ Directory arguments are expanded to all contained files matching the glob
     )
 }
 
+fn parallel_arg(descr: &str, jobs_fallback: bool) -> Arg {
+  let arg = Arg::new("parallel")
+    .long("parallel")
+    .help(format!("Run {descr} in parallel. Parallelism defaults to the number of available CPUs or the value in the DENO_JOBS environment variable."))
+    .action(ArgAction::SetTrue);
+  if jobs_fallback {
+    arg.conflicts_with("jobs")
+  } else {
+    arg
+  }
+}
+
 fn types_subcommand() -> Command {
   Command::new("types").about(
     "Print runtime TypeScript declarations.
@@ -4416,6 +4429,8 @@ fn serve_parse(
     .remove_one::<String>("host")
     .unwrap_or_else(|| "0.0.0.0".to_owned());
 
+  let worker_count = parallel_arg_parse(matches, false).map(|v| v.get());
+
   runtime_args_parse(flags, matches, true, true);
   // If the user didn't pass --allow-net, add this port to the network
   // allowlist. If the host is 0.0.0.0, we add :{port} and allow the same network perms
@@ -4455,6 +4470,7 @@ fn serve_parse(
     watch: watch_arg_parse_with_paths(matches),
     port,
     host,
+    worker_count,
   });
 
   Ok(())
@@ -4486,6 +4502,42 @@ fn task_parse(flags: &mut Flags, matches: &mut ArgMatches) {
   flags.subcommand = DenoSubcommand::Task(task_flags);
 }
 
+fn parallel_arg_parse(
+  matches: &mut ArgMatches,
+  fallback_to_jobs: bool,
+) -> Option<NonZeroUsize> {
+  if matches.get_flag("parallel") {
+    if let Ok(value) = env::var("DENO_JOBS") {
+      value.parse::<NonZeroUsize>().ok()
+    } else {
+      std::thread::available_parallelism().ok()
+    }
+  } else if fallback_to_jobs && matches.contains_id("jobs") {
+    // We can't change this to use the log crate because its not configured
+    // yet at this point since the flags haven't been parsed. This flag is
+    // deprecated though so it's not worth changing the code to use the log
+    // crate here and this is only done for testing anyway.
+    #[allow(clippy::print_stderr)]
+    {
+      eprintln!(
+        "⚠️ {}",
+        crate::colors::yellow(concat!(
+          "The `--jobs` flag is deprecated and will be removed in Deno 2.0.\n",
+          "Use the `--parallel` flag with possibly the `DENO_JOBS` environment variable instead.\n",
+          "Learn more at: https://docs.deno.com/runtime/manual/basics/env_variables"
+        )),
+      );
+    }
+    if let Some(value) = matches.remove_one::<NonZeroUsize>("jobs") {
+      Some(value)
+    } else {
+      std::thread::available_parallelism().ok()
+    }
+  } else {
+    None
+  }
+}
+
 fn test_parse(flags: &mut Flags, matches: &mut ArgMatches) {
   flags.type_check_mode = TypeCheckMode::Local;
   runtime_args_parse(flags, matches, true, true);
@@ -4552,36 +4604,7 @@ fn test_parse(flags: &mut Flags, matches: &mut ArgMatches) {
     flags.argv.extend(script_arg);
   }
 
-  let concurrent_jobs = if matches.get_flag("parallel") {
-    if let Ok(value) = env::var("DENO_JOBS") {
-      value.parse::<NonZeroUsize>().ok()
-    } else {
-      std::thread::available_parallelism().ok()
-    }
-  } else if matches.contains_id("jobs") {
-    // We can't change this to use the log crate because its not configured
-    // yet at this point since the flags haven't been parsed. This flag is
-    // deprecated though so it's not worth changing the code to use the log
-    // crate here and this is only done for testing anyway.
-    #[allow(clippy::print_stderr)]
-    {
-      eprintln!(
-        "⚠️ {}",
-        crate::colors::yellow(concat!(
-        "The `--jobs` flag is deprecated and will be removed in Deno 2.0.\n",
-        "Use the `--parallel` flag with possibly the `DENO_JOBS` environment variable instead.\n",
-        "Learn more at: https://docs.deno.com/runtime/manual/basics/env_variables"
-        )),
-      );
-    }
-    if let Some(value) = matches.remove_one::<NonZeroUsize>("jobs") {
-      Some(value)
-    } else {
-      std::thread::available_parallelism().ok()
-    }
-  } else {
-    None
-  };
+  let concurrent_jobs = parallel_arg_parse(matches, true);
 
   let include = if let Some(files) = matches.remove_many::<String>("files") {
     files.collect()