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, 8 insertions, 4 deletions

diff --git a/ext/net/01_net.js b/ext/net/01_net.js
index 517ab127e..536f79bbf 100644
--- a/ext/net/01_net.js
+++ b/ext/net/01_net.js
@@ -531,10 +531,14 @@ const listenOptionApiName = Symbol("listenOptionApiName");
 function listen(args) {
   switch (args.transport ?? "tcp") {
     case "tcp": {
-      const { 0: rid, 1: addr } = op_net_listen_tcp({
-        hostname: args.hostname ?? "0.0.0.0",
-        port: Number(args.port),
-      }, args.reusePort);
+      const { 0: rid, 1: addr } = op_net_listen_tcp(
+        {
+          hostname: args.hostname ?? "0.0.0.0",
+          port: Number(args.port),
+        },
+        args.reusePort,
+        args.loadBalanced ?? false,
+      );
       addr.transport = "tcp";
       return new Listener(rid, addr);
     }