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.
We can land it now, I'll put together a followup and we'll just not ship a patch release until the followup is in. Sound good?