Comment Doc directives colouring and completion

[FMorschel]

In Dart, we have things like comment references that are highlighted with the same colour as the declaration. This helps the user know that part of the comment is not comment-like but will be shown in the comment.

I'd like to request that we colour Dart doc directives too. So things like {@template ...}, {@macro ...} and similar are differentiated for the user. And probably completion too, since this would align with the doc_directive_unknown warning since it already triggers for invalid values.

I'd say they could be colored as annotations, but this is just a suggestion.

CC @srawlins @bwilkerson

[bwilkerson]

I suspect that the way this would work for LSP is that we'd continue to return a token type of SemanticTokenTypes.comment but would use a new custom modifier of something like dartDocDirective. The color would then be the same as the rest of the comment but could be overridden by the user.

@DanTup To see how close I am.

To support this for IntelliJ we'd need to update the protocol to support the new HighlightRegionType, then update the plugin to support the new type.

[DanTup]

@DanTup To see how close I am.

Yep, sounds right to me :-)