feat: Allow "var(--" strings as values for all properties

[nmn]

What changed / motivation ?

CSS properties are typed in StyleX, and while some properties allow arbitrary strings, many don't. These properties should still permit usages of string referencing CSS variables. The new StringPrefix<> type in Flow makes this possible.

[github-actions]

compressed-size: runtime library

Size change: +0.00 kB
Total size: 2.52 kB

Filename: gzip (minify)	kB size	kB change	% change
./packages/stylex/lib/StyleXSheet.js	1.48 (3.75)	+0.00 (+0.00)	+0.1% (+0.1%)

View unchanged

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%)

[github-actions]

compressed-size: e2e bundles

Size change: 0.00 kB
Total size: 1125.68 kB

View unchanged

Filename: gzip (minify)	kB size	kB change	% change
./apps/rollup-example/.build/bundle.js	1002.49 (10185.35)	0.00 (0.00)	0.0% (0.0%)
./apps/rollup-example/.build/stylex.css	123.19 (774.43)	0.00 (0.00)	0.0% (0.0%)

[necolas]

We need so many FlowFixMe to use this API 🥲

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.

@stylexjs/[email protected] size:compare
./size-compare.js /tmp/tmp.yZNU1UH0mS /tmp/tmp.JKLLiRdqtC

Results	Base	Patch	Ratio
stylex/lib/stylex.js
· compressed	899	899	1.00
· minified	3,223	3,223	1.00
stylex/lib/StyleXSheet.js
· compressed	1,266	1,266	1.00
· minified	3,776	3,776	1.00
rollup-example/.build/bundle.js
· compressed	563,484	563,484	1.00
· minified	10,185,368	10,185,368	1.00
rollup-example/.build/stylex.css
· compressed	99,154	99,154	1.00
· minified	745,649	745,649	1.00

[nmn]

NOTE: this will be merged after verifying how it affects our code internally.

[nmn]

Update: The StringPrefix type still isn't ready. It has some strange bugs when being used for reads. So, this PR will have to wait for now.