Extend CSS color support with css-color module

[asamuzaK]

Currently, support for CSS colors is very limited.
The css-color module covers most of the colors in CSS color module level 4.

NOTE: Colors are converted to specified value, not computed value.

NOTE2: In the test, caching seems to be working.
For this reason, I think the elapsed time displayed in test results are not so reliable.

If I add the same test, elapsed time significantly decreased.

  ✔ should normalize color-mix values (217.8072ms)
  ✔ should normalize color-mix values (2nd attempt) (61.3616ms)
  ✔ should normalize color-mix values (3rd attempt) (0.3769ms)

Fixes #134, fixes #135, fixes #136, fixes #174

[asamuzaK]

PR ready.
I think we can probably delete lib/named_colors.json as well, but I'm not sure, so I left it as is.

[domenic]

Very nice, thank you!