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, 37 insertions, 3 deletions

diff --git a/runtime/worker_bootstrap.rs b/runtime/worker_bootstrap.rs
index b13c3c428..afd3242e8 100644
--- a/runtime/worker_bootstrap.rs
+++ b/runtime/worker_bootstrap.rs
@@ -10,7 +10,6 @@ use deno_terminal::colors;
 
 /// The execution mode for this worker. Some modes may have implicit behaviour.
 #[derive(Copy, Clone)]
-#[repr(u8)]
 pub enum WorkerExecutionMode {
   /// No special behaviour.
   None,
@@ -28,11 +27,39 @@ pub enum WorkerExecutionMode {
   /// `deno bench`
   Bench,
   /// `deno serve`
-  Serve,
+  Serve {
+    is_main: bool,
+    worker_count: Option<usize>,
+  },
   /// `deno jupyter`
   Jupyter,
 }
 
+impl WorkerExecutionMode {
+  pub fn discriminant(&self) -> u8 {
+    match self {
+      WorkerExecutionMode::None => 0,
+      WorkerExecutionMode::Worker => 1,
+      WorkerExecutionMode::Run => 2,
+      WorkerExecutionMode::Repl => 3,
+      WorkerExecutionMode::Eval => 4,
+      WorkerExecutionMode::Test => 5,
+      WorkerExecutionMode::Bench => 6,
+      WorkerExecutionMode::Serve { .. } => 7,
+      WorkerExecutionMode::Jupyter => 8,
+    }
+  }
+  pub fn serve_info(&self) -> (Option<bool>, Option<usize>) {
+    match *self {
+      WorkerExecutionMode::Serve {
+        is_main,
+        worker_count,
+      } => (Some(is_main), worker_count),
+      _ => (None, None),
+    }
+  }
+}
+
 /// The log level to use when printing diagnostic log messages, warnings,
 /// or errors in the worker.
 ///
@@ -175,6 +202,10 @@ struct BootstrapV8<'a>(
   u16,
   // serve host
   Option<&'a str>,
+  // serve is main
+  Option<bool>,
+  // serve worker count
+  Option<usize>,
 );
 
 impl BootstrapOptions {
@@ -186,6 +217,7 @@ impl BootstrapOptions {
     let scope = RefCell::new(scope);
     let ser = deno_core::serde_v8::Serializer::new(&scope);
 
+    let (serve_is_main, serve_worker_count) = self.mode.serve_info();
     let bootstrap = BootstrapV8(
       self.location.as_ref().map(|l| l.as_str()),
       self.unstable,
@@ -198,9 +230,11 @@ impl BootstrapOptions {
       self.disable_deprecated_api_warning,
       self.verbose_deprecated_api_warning,
       self.future,
-      self.mode as u8 as _,
+      self.mode.discriminant() as _,
       self.serve_port.unwrap_or_default(),
       self.serve_host.as_deref(),
+      serve_is_main,
+      serve_worker_count,
     );
 
     bootstrap.serialize(ser).unwrap()