Support user OPT defined constant character sets

[tobiasvl]

Users can define their own character sets for use in some constant types with the OPT directive. See documentation.

I'm not sure how easy that is to implement. This Vim syntax file uses a naïve approach.

[georgjz]

I don't think that's possible with regex, since it would require some kind of recursive lookup. But it might be once the tree-sitter support for Atom works better.