fix: Process two-value shorthands appropriately

[mellyeliu]

Context

Previously we were indiscriminately expanding all multi-value shorthands to all four directions. Let's clean this up and split two-value shorthands like:

• margin: '10em 1em' -> marginInline: '10em' and marginBlock: '1em'
• padding: '10em 1em' -> paddingInline: '10em' and paddingBlock: '1em'

Implementation

Check for number of values after splitting:

• If 1, return no error as before
• If 2, return inline and block values
• Else, return four directions

Testing

Added in test cases to ensure that two-value shorthands are accounted for.

• I have read the contributing guidelines
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• Performed a self-review of my code

[github-actions]

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]

compressed-size: e2e bundles

Size change: 0.00 kB
Total size: 1125.68 kB

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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%)