syntax highlighters implementation for enhanced enums

[devoncarew]

This is the syntax highlighters implementation for enhanced enums.

• https://github.com/dart-lang/dart-syntax-highlight/
• https://github.com/highlightjs/highlight.js
• https://github.com/codemirror/CodeMirror/

See #47849 for the main tracking issue.

[itsjustkevin]

@devoncarew who should we contact to confirm the status of this issue?

[devoncarew]

@DanTup, @srawlins, and @domesticmouse, respectively :)

[srawlins]

For highlight.js, enhanced-enums seems to work out of the box.

[domesticmouse]

Plugging the following into DartPad syntax highlights corrrectly:

enum Ordering with EnumIndexOrdering<Ordering> {
  zero,
  few,
  many;
}

mixin EnumIndexOrdering<T extends Enum> on Enum implements Comparable<T> {
  @override
  int compareTo(T other) => index - other.index;
}

void main() {
  var lst = [Ordering.many, Ordering.zero, Ordering.few]..sort();
  print(lst);
}

[DanTup]

The example above looks good to me in both the Textmate grammar and LSP too (the difference in white/blue is expected because the textmate grammar leaves them uncoloured but semantic tokens applies tokens).

Textmate grammar:

LSP Semantic Tokens: