Enable setting a single var from stylex.defineVars

[nmn]

@necolas mentioned that when using the workaround for descendent selector, the variable should come from defineVars. This adds basic support for doing just that.

Here are the current limitations:

1. defineVars still needs to be in a separate .stylex.js file.
   • It should be possible to allow using defineVars inline within a file as long as it is only used within stylex.create or stylex.createTheme calls. No exports.
   • But, any use-cases for defining variables in the same file will be solved with better solutions.
2. The ESlint rule only allows an object value
   • It might make sense to verify the value is a string at the very least as it's not clear what a number value should be.

[github-actions]

compressed-size: runtime library

Size change: 0.00 kB
Total size: 2.36 kB

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Filename: gzip (minify)	kB size	kB change	% change
./packages/stylex/lib/stylex.js	1.02 (3.53)	0.00 (0.00)	0.0% (0.0%)
./packages/stylex/lib/StyleXSheet.js	1.34 (3.41)	0.00 (0.00)	0.0% (0.0%)

[github-actions]

compressed-size: e2e bundles

Size change: 0.00 kB
Total size: 1119.39 kB

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Filename: gzip (minify)	kB size	kB change	% change
./apps/rollup-example/.build/bundle.js	1005.22 (9872.52)	0.00 (0.00)	0.0% (0.0%)
./apps/rollup-example/.build/stylex.css	114.18 (455.95)	0.00 (0.00)	0.0% (0.0%)