fix: build .mjs functions with esbuild

[eduardoboucas]

- Summary

Since netlify/zip-it-and-ship-it#619, functions can have a .mjs entry point. With this PR, CLI will ensure that those functions are built with esbuild as part of netlify dev, just like TypeScript functions.

Closes #3252.

- Test plan

Added a new test.

- A picture of a cute animal (not mandatory but encouraged)

[github-actions]

📊 Benchmark results

Comparing with f5743f0

Package size: 327 MB

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• T-30 (f5743f0): 327 MB
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• T-10 (f977a77): 327 MB
• T-9 (cc84d2a): 327 MB
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• T-3 (aa522cb): 327 MB
• T-2 (f548f2b): 327 MB
• T-1 (33d0208): 327 MB
• T (current commit): 327 MB