feat(child_process): add node:child_process polyfill

[ThatOneBro]

Beginning of node:child_process polyfill, adds base case for spawn without any extra options specified, and the necessary functionality of the ChildProcess class.

[Jarred-Sumner]

Great start!

[ThatOneBro]

Most of spawn parameters are now implemented, now porting all node tests to make sure everything is actually to spec. spawnSync is next which should be mostly similar except the interfaces (and the fact that it returns synchronously, obviously). The rest of the exports from child_process are just different flavored wrappers around spawn and spawnSync.

However after this, we will still lack IPC functionality. This is because we still need to add a check for a NODE_CHANNEL_FD env var and conditionally call to _forkSync at the beginning of every Bun process after spawning. This opens the channel on the child process side which is required for the two way communication.

Many of the important use cases of child_process can probably operate without IPC so we might want to merge this branch after all packages but fork are working. If that makes sense to do so.

I will give another update later after I get the node tests running and finish up spawnSync and the rest of the easy to finish exports. 👍

[Jarred-Sumner]

these might need to be getters so that the values are the same/not copied but i'm not 100% sure

[Jarred-Sumner]

these might need to be getters so that the values are in sync if the user assigns it but i'm not 100% sure

[Jarred-Sumner]

I'm going to go ahead and merge this! Great work.

If the spawnSync issue isn't fixed we can fix it in a follow-up PR.