fix: Don't add units number values used for variables

[nmn]

fixes #689

For convenience and to match the behaviour of React Native, StyleX allows the usage of numbers as values and tries to automatically add units where they make sense.

The issue, however, is that for any unknown value StyleX assumes it to be a length and adds px unit by default. While we should probably revisit that behaviour itself, for now, this PR adds a special case for CSS variables and leaves numbers used as values for CSS variables keys as is.

[github-actions]

workflow: benchmarks/size

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

@stylexjs/scripts@0.7.5 size:compare
./size-compare.js /tmp/tmp.Gh8zrY4Y1I /tmp/tmp.XEZolNWkkx

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	563,563	563,563	1.00
· minified	10,185,348	10,185,348	1.00
rollup-example/.build/stylex.css
· compressed	100,211	100,211	1.00
· minified	759,868	759,868	1.00