Initial implementation of js-post-build in rewatch

[nojaf]

Fixes #8150

It does use the absolute path for now.

[pkg-pr-new]

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rescript

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@rescript/darwin-arm64

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@rescript/darwin-x64

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@rescript/linux-arm64

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@rescript/linux-x64

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@rescript/runtime

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@rescript/win32-x64

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commit: ac4efda

[nojaf]

@cknitt any thoughts on how big of a deal the absolute path is here?

[cknitt]

I agree that passing an absolute path should be clearer.
I have never used this feature in practice though.
Maybe ask in the forum post for feedback? https://forum.rescript-lang.org/t/rescript-v12-js-post-build-confusion/7053