Support escaped backticks in template literals

This is a totally valid use but since the lexer just looks for the next
backtick as a marker for the end of the literal, things fall apart
quickly.

I added this use to the fixture, as well as one for literals without templates... just in case.
I don't really care and honestly feel like that's a decision you should make outside of this PR. Is there anything else I needed to do here?