Refactor synIDattr into is_string
BUG: Separate matched vs unmatched pairs

BUG: This breaks an additional test for quotes. Not sure why.

Reduce the duplication of the pair characters by separating the two
types into dicts that we can join into text classes.
Add support for angle brackets
Junk: Try to make string checking optional

Trying to be able to flip to the original (stackoverflow)
implementation, but I'm missing something because it doesn't work.