Get occurrences for 'this' keywords

@ahejlsberg and I felt like renaming `this` isn't so unheard of, but yes, it is currently broken in this implementation - rename will populate the initial text with the symbol name in VS.

What semantics did you have for findAllRefs on `this`? While we don't go out of our way to match `this` across merged declarations, we certainly have fairly predictable results. Thoughts @mhegazy, as we discussed this offline?