[misc] vim/syntax: Fixups

[misc] vim/syntax: Fixups

(Thanks, @mrjbq7!) Now:
+ `CHAR:` literals highlight the whole next token.
+ `0b...` binary literals don't require invalid `+=0b` or `-=0b` syntax.
+ Float literals can't start with a `,` separator.
+ Float literals can have exponents with `,` separators.
+ `foo: ...` stack effects function as intended in general.
+ Syntax clusters might be a bit cleaner with `g:factor_syn_no_error`.