chore: Add cjs to rollup-plugin

[VojGin]

What changed / motivation ?

I had came to a need of stylex rollup-plugin in the cjs flavor. See this issue for relevant context. https://github.com/facebook/stylex/issues/451

So I added it. My PR is very much inspired by the current implementation in packages/styles/package.json

Linked PR/Issues

Fixes https://github.com/facebook/stylex/issues/451

Additional Context

The rollup-plugin now generates lib/* and lib/es/* folders. See screenshot:

I had run all the tests and lints. But as I'm unfamiliar with the repository. I'd gladly ask for a double check that I didn't break anything vital. 🙏

What I have manually tested:

• The output lib/es/index.mjs is identical with the old lib/index.js
• The cjs flavor of the library was tested in my project and seems to be building correctly

Tagging @nmn as a follow-up to our discussion

Pre-flight checklist

• [x] I have read the contributing guidelines Contribution Guidelines
• [x] Performed a self-review of my code

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[nmn]

Needing cjs with Rollup was not something I would've expected. Will take a look at this soon.

[Luko248]

@VojGin This is exactly what we currently need on our project. @nmn What is blocking this merge request please?

[nmn]

I was away for a couple of weeks. Thanks for the PR! Will put out a release soon.