Add highlight for keywords

[gjvnq]

Hi, I think that the special rules should have their own highlight.

I think adding this bit of code would suffice:

- match: '\b(ANY|SOI|EOI|WHITESPACE|COMMENT|PUSH|POP)\b'
      scope: keyword

What do you think?

[dten]

I don't think keyword is quite right. And this would make navigation to their definition (in the case of whitespace and comment) not work.
I'll have a think. Probably one of the support types is better

[giraffekey]

I get highlights for ASCII_ALPHANUMERIC, ANY, NEWLINE, etc, but not for ASCII_ALPHA, ASCII_DIGIT, or various others.

To stay consistent I think everything mentioned on this page should be highlighted: https://pest.rs/book/grammars/built-ins.html