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, 10 insertions, 6 deletions

diff --git a/cli/tools/run/mod.rs b/cli/tools/run/mod.rs
index 65044fbad..1964cfdd9 100644
--- a/cli/tools/run/mod.rs
+++ b/cli/tools/run/mod.rs
@@ -18,11 +18,7 @@ use crate::util::file_watcher::WatcherRestartMode;
 
 pub mod hmr;
 
-pub async fn run_script(
-  mode: WorkerExecutionMode,
-  flags: Arc<Flags>,
-  watch: Option<WatchFlagsWithPaths>,
-) -> Result<i32, AnyError> {
+pub fn check_permission_before_script(flags: &Flags) {
   if !flags.has_permission() && flags.has_permission_in_argv() {
     log::warn!(
       "{}",
@@ -33,6 +29,14 @@ To grant permissions, set them before the script argument. For example:
       )
     );
   }
+}
+
+pub async fn run_script(
+  mode: WorkerExecutionMode,
+  flags: Arc<Flags>,
+  watch: Option<WatchFlagsWithPaths>,
+) -> Result<i32, AnyError> {
+  check_permission_before_script(&flags);
 
   if let Some(watch_flags) = watch {
     return run_with_watch(mode, flags, watch_flags).await;
@@ -187,7 +191,7 @@ pub async fn eval_command(
   Ok(exit_code)
 }
 
-async fn maybe_npm_install(factory: &CliFactory) -> Result<(), AnyError> {
+pub async fn maybe_npm_install(factory: &CliFactory) -> Result<(), AnyError> {
   // ensure an "npm install" is done if the user has explicitly
   // opted into using a managed node_modules directory
   if factory.cli_options()?.node_modules_dir_enablement() == Some(true) {