std/node: add child_process module

[EvanHahn]

For completeness, we should add a polyfill for Node's child_process module.

[cheney-enterprises]

@bartlomieju @EvanHahn I am working on child_process, and cleaning up process to be more compatible with child_process and the original node api. It may be awhile yet due to time constraints but hoping to get this up and running.

[stale]

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