feat(child_process): add node:child_process polyfill (#1424)

* feat(child_process): beginning of child_process, add ChildProcess and spawn base case

* fix(child_process): remove invalid single arg array syntax (thanks Copilot)

* refactor(child_process): unhack Readable.on, move stuff into node:stream

* feat(child_process): add more params for spawn, refactor, add fromWeb() to Readable

* feat(child_process): finish rest of exports (minus fork), refactor, add tests

* cleanup(streams): remove a bunch of unnecessary stuff

* cleanup(child_process): remove dead refs

* fix(child_process): fix stdio

* fix(child_process): change stdio to bunStdio

* test(child_process): uncomment timeout test

* test(child_process): fix hanging tests

* test(child_process): remove stray console.log

* test(child_process): fix cwd test for linux

* refactor(child_process): divide paths for encoded vs raw execFile stdio

* fix(child_process): fix logic for execFile slow path