Modernize the CSS lexer

[jneen]

Supersedes #2150.

• Adds rebeccapurple to the colour list
• Reduces the warnings from [https://bugs.ruby-lang.org/issues/21870](the warning spam bug) to one
• Supports CSS nesting
• Colours the contents of url(...) and url-prefix(...) as Str::Other even if they are unquoted.

TODO:

• Automatically sync the keywords and colours here, ideally from W3 directly, and factor it out into a lazy-loaded file.
• More explicit support for custom properties and calc()

[petamoriken]

Automatically sync the keywords and colours here, ideally from W3 directly, and factor it out into a lazy-loaded file.

While we could wait for the keywords to be automatically updated, it would be much appreciated if you could add flow and flow-root, which are used in "display multi-keyword syantax."
https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Display/Multi-keyword_syntax#using_the_multi-keyword_syntax

[petamoriken]

It would be better to have a test case where a media query is nested within CSS Nesting.

.container {
  padding: 10px;

  @media (width > 768px) {
    padding: 20px;
  }
}

[jneen]

Good catch! I'll add that to the spec.

[jneen]