2.10 improvements - highlight ???
Restrict { } and | characters in identifiers

It's technically legal to use characters like } and | in class, method,
and variable names, but in practice they must be wrapped in backticks
and are not common anyway. Removing them in the syntax file makes string
interpolation highlighting more sane.
2.10 improvement - String interpolation in braces

This supports "foo ${bar} baz" style interpolation, with braces.
Not "foo $bar baz" yet.
Include 'fixme' as a todo-highlight