Supporting CSS Color Module Level 4 and 5

[JakeQZ]

This is a task list of enhancements required to fully support the above (strictly).

High priority (ideally included in next release)

• Allow interchangeability of rgb and rgba, and hsl and hsla ([FEATURE] Support rgba and rgb (etc.) being aliases #797)
• Parse color functions which use the "modern" syntax ([FEATURE] Parse color functions with modern syntax #800)
• Support the none keywod in the "modern" syntax
• Add tests to confirm that all varieties of angle are permitted for hue ([TASK] Add TestCase for Value\Color #793)
• Add tests to confirm that both number and percentage are permitted where allowed (i.e. in "modern" rgb syntax) ([BUGFIX] Don't render rgb colors with % values as hex #803)

Low priority (would be nice to have, but we have other fish to fry in the meantime, so to speak)

• Add rendering option to output color functions using "modern" syntax
• Reject color functions with invalid length arguments, such as 10px, including rejecting percentage arguments where these are not permitted (e.g. in "legacy" syntax)
• Parse color functions containing var() substitutions as Color rather than generic CSSFunction
• Support additional color functions (these will currently be parsed as generic CSS functions, which may not work reliably):
  • hwb
  • lab
  • lch
  • oklab
  • oklch
• Support color function:
  • Level 4
  • Level 5
• Support relative colors (level 5)