[stylex] improve treeshakability

[henryqdineen]

What changed / motivation ?

This PR makes a couple changes to improve the treeshakability of the @stylexjs/stylex package.

• Set sideEffects flag to false in the package.json. Although rare in typical usage this would allow bundlers to remove unused imports from '@stylexjs/stylex'. This is also a general best practice if the library has no side effects. I noticed that styleq already has this flag set
• Moved the code that patches properties onto legacyMerge into a IIFE and annotated it with /*@__PURE__*/ with a rollup plugin. Even with sideEffect: false set some bundlers (definitely webpack from my testing) would consider setting a property like _legacyMerge.create = to be a side effect and the code would not be marked for dead code elimination. I tried adding/*@__PURE__*/ in the source but it was not being preserved, so I resorted to a rollup plugin. I think there is something I couldn't easily figure out about the flow or hermes-parser plugins that remove comments.

With webpack and this change I am seeing a bundle size reduction from 3.4KB to 1.4KB for a simple entry point that only uses the props import (webpack is able to analyze namespace imports in most cases)

import * as stylex from '@stylexjs/stylex';
console.log(stylex.props)

Questions

• Can we just drop legacyMerge or at least the patching of the variables onto it? It seems undocumented?

Pre-flight checklist

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

[nmn]

Can we just drop legacyMerge or at least the patching of the variables onto it? It seems undocumented?

We're already discussing this...