[augmentations] Syntax highlighters implementation

[itsjustkevin]

[dart-github-bot]

Summary: The issue concerns the implementation of syntax highlighters for augmentations, which are a new feature in Dart that allows for extending existing classes and libraries.

[keertip]

@DanTup, where would we track the VS code work?

[DanTup]

We can have an issue for it here, or in Dart-Code. I suspect any changes to syntax highlighting will just be in the analysis_server/LSP mappings, and maybe also in the dart-syntax-highlighting repo (which I think we usually just track in issues here in the SDK) which will then just need rolling into Dart-Code.

(usually for these kinds of issues we have a checkbox list of the syntax highlighters - for example see #56187 - maybe we should add this here too?). Edit: And in #56196?