Fix buggy syntax highlighting behavior involving "?"

Fixes #42 
Fixes #29 
Fixes #30 
Fixes #36 
Refs #23 

These fixes all stem from removing `elixir-syntax-propertize` and instead relying on the regex creation work in `elixir-rx` and `elixir-rx-constituents`. In short, by adding a font face to the `code-point` regex constituent, and putting it last in the list of defaults, it ensures the "escape character" fontification for the question mark will only be used as a last resort.

Regarding #23, that is still a failing test. I think there will need to be some further use of `rx` to ensure that the `"` character does not override the `~r//` pattern. That is, a regex closure should signal the end of any previous fontification. It doesn't now, but it should.