Syntax highlighting for all enum-like UDAs

[claui]

As a generalization of #12, I could do the same for any UDA that behaves like an enum, not just priority.

A possible implementation would register with VS Code’s semantic highlighting/semantic tokens provider API, and internally do something akin to:

$ task _show | grep 'uda\.[^.]*\.values='

Note that the task executable may be unavailable. Even if it is available, it may take a while until VS Code decides to invoke the provider because it’s busy doing other things.
Either way, the scopes added in #12 for the special case of priority are still helpful as a fallback, so I’d like to keep them around.