Feat: Big improvements to `stylex.defineVars` and `stylex.createTheme`

[nmn]

What changed / motivation ?

• [x] stylex.defineVars and stylex.createTheme will now correctly handle arbitrarily nested at rules within value. It previously accepted only a flat object.
• [x] stylex.createTheme should generate a unique class name given all given values. Previously it only accounted for the default values of styles when creating the hash.
• [x] Add additional tests for new behaviour
  • [x] Add tests for multiple levels of nested at-rules. (Combine @media and @supports)
  • [x] Add tests for two themes with the same default values but different values within at-rules.
• [x] Add support for using stylex.types.* functions within stylex.defineVars and stylex.createTheme.
• [x] Fix TypeScript issues. It's too permissive at the moment.

[github-actions[bot]]

compressed-size: runtime library

Size change: 0.00 kB Total size: 2.52 kB

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| Filename: gzip (minify) | kB size | kB change | % change | | :--- | :--- | :--- | :--- | | `./packages/stylex/lib/stylex.js` | **1.04** (3.22) | **0.00** (0.00) | **0.0%** (0.0%) | | `./packages/stylex/lib/StyleXSheet.js` | **1.48** (3.75) | **0.00** (0.00) | **0.0%** (0.0%) |

[github-actions[bot]]

compressed-size: e2e bundles

Size change: 0.00 kB Total size: 1128.55 kB

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| Filename: gzip (minify) | kB size | kB change | % change | | :--- | :--- | :--- | :--- | | `./apps/rollup-example/.build/bundle.js` | **1005.20** (10185.36) | **0.00** (0.00) | **0.0%** (0.0%) | | `./apps/rollup-example/.build/stylex.css` | **123.34** (773.82) | **0.00** (0.00) | **0.0%** (0.0%) |