Update grammar to reflect the ability to use reserved keywords in annotation names

[patjakdev]

This line states that annotation names should be IDENTs, which was true until this PR was merged.

The grammar should now allow any IDENT | RESERVED there. Or maybe the new AnyId concept in the Rust parser could be immortalized in the grammar.

[khieta]

Adding ANYIDENT seems like the cleanest solution to me, but open to feedback on #132