fix: ensure unique classNames for pseudo classes & elems

[nmn]

What changed

We recently fixed an issue with how pseudo selectors are sorted in StyleX to ensure we don't accidentally mix up pseudo classes and pseudo elements incorrectly.

However, while we fixed the generated rules in the previous PR, the string hashed to generate a unique className was still using the old logic. This would mean that, e.g. if you used both ::before:hover and :hover::before selectors in your styles for the exact same property and value, both rules would generate the same className.

(TODO: Create a separate commit to actually demonstrate this edge-case in a test)

This PR updates the logic used when generating the className to match the logic used when generating the CSS rule to fix this issue.

Fixes #749

[github-actions]

workflow: benchmarks/size

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

@stylexjs/scripts@0.8.0 size:compare
./size-compare.js /tmp/tmp.LuB14pXxuu /tmp/tmp.lh3ryT1kwc

Results	Base	Patch	Ratio
stylex/lib/stylex.js
· compressed	729	729	1.00
· minified	2,541	2,541	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	563,025	563,025	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