Introduce a fixed set of syntax highlighting categories to replace the old string based categories in parse trees

[DavyLandman]

Right now people writing a grammar can add @Category annotations on their productions to get syntax highlighting in their IDE. It's been a bit of a mixed bag of what to write vs what you'll get. usethesource/rascal-language-servers#366 was a recent case of this misalignment.

After some more discussions with @jurgenvinju we came to the following design:

• ParseTree gets a new ADT called Highlight
• We use a fixed set of highlight categories (that happen to mirror the LSP SemanticTokenType categories)
• Production rules can now be annotated with either the deprecated @Category or the new @Highlight=number()

[jurgenvinju]

Thanks Davy. Minor note: the coding standard for tags is to start with lowercase, and for ADTs with uppercase. This helps to pry apart Highlight the ADT from @highlight=keyword() the tag name. Just like @synopsis{...}