fix(node): Rework node:child_process IPC (#24763)

Fixes https://github.com/denoland/deno/issues/24756. Fixes
https://github.com/denoland/deno/issues/24796.

This also gets vitest working when using
[`--pool=forks`](https://vitest.dev/guide/improving-performance#pool)
(which is the default as of vitest 2.0). Ref
https://github.com/denoland/deno/issues/23882.

---

This PR resolves a handful of issues with child_process IPC. In
particular:

- We didn't support sending typed array views over IPC
- Opening an IPC channel resulted in the event loop never exiting
- Sending a `null` over IPC would terminate the channel
- There was some UB in the read implementation (transmuting an `&[u8]`
to `&mut [u8]`)
- The `send` method wasn't returning anything, so there was no way to
signal backpressure (this also resulted in the benchmark
`child_process_ipc.mjs` being misleading, as it tried to respect
backpressure. That gave node much worse results at larger message sizes,
and gave us much worse results at smaller message sizes).
- We weren't setting up the `channel` property on the `process` global
(or on the `ChildProcess` object), and also didn't have a way to
ref/unref the channel
- Calling `kill` multiple times (or disconnecting the channel, then
calling kill) would throw an error
- Node couldn't spawn a deno subprocess and communicate with it over IPC

ext/node/benchmarks/child_process_ipc.mjs

@@ -5,10 +5,20 @@ import { setImmediate } from "node:timers";
 if (process.env.CHILD) {
   const len = +process.env.CHILD;
   const msg = ".".repeat(len);
+  let waiting = false;
   const send = () => {
-    while (process.send(msg));
+    while (
+      process.send(msg, undefined, undefined, (_e) => {
+        if (waiting) {
+          waiting = false;
+          setImmediate(send);
+        }
+      })
+    );
     // Wait: backlog of unsent messages exceeds threshold
-    setImmediate(send);
+    // once the message is sent, the callback will be called
+    // and we'll resume
+    waiting = true;
   };
   send();
 } else {