Use a better CSS value parser to normalize values

[nmn]

We currently use postcss-value-parser to parse the values used in stylex.create and normalize them. However, this is a basic parser and we're mostly limited in the normalisation we can perform to being able to strip extra white space, and sometimes drop units used with 0.

We can use a more sophisticated CSS value parser that understand CSS value types to be able to normalise many more kinds of values.

Here are a few examples:

• rgb(0 0 0 / 50%) should be normalised to rgba(0,0,0,0.5)
• #000000 should be normalised to #000
• We should either convert all named CSS colours to hashes or convert hash values to named values when possible.
• @media (width >= 768px) should be normalised to @media (min-width: 768px)
• etc.

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nmn/css-value-parser is a fairly full featured parser that I worked on specifically for this purpose.

We could also consider using lightningcss if that is a good fit.