Support Relative Alpha Colors(CSS Color 5 alpha() function)

[yisibl]

I was involved in the development of alpha() in Chrome, so once it’s released in Chrome, I’ll start working on implementing it in LightningCSS.

• Spec: https://drafts.csswg.org/css-color-5/#relative-alpha
• Chrome: https://groups.google.com/a/chromium.org/g/blink-dev/c/fl5TIBDLHuo
• WebKit: [Color] Add support for the alpha() relative color function WebKit/WebKit#66126
• Firefox bug 2042646
• Relative Alpha Colors mozilla/standards-positions#1396 (comment)