GDScript: Add abstract functions to highlight grammar

[HolonProduction]

Fixes: #962

Also adds is not (although not sure on the role, but it looks fine with the default color scheme)
Fixes: #834

[DaelonSuzuka]

@HolonProduction FYI I haven't been particularly strict on the token roles, there's a few contributors that come and correct me every once in a while.

The textmate tokenizer IS used for the formatter though (don't ask), so if you do something too outrageous with the highlighting the formatter tests should tell you. (this also means that when there's new grammar in GDScript, it's doubly helpful to make sure it's represented in the tests, because that checks both the formatter and the highlighting [indirectly])