Add support for development on Windows

[code-asher]

• Use cross-env
• Spawns in watch.ts don't work for some reason.
• May want to port the build scripts to JS for portability (this could also make them easier to unit test if we decide to do that).

See this discussion for more information: #3488

[jsjoeio]

Related: #3554 (comment)

[stale]

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