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.
This looks okay to me (from a glance) --- we might want to further generalize this to distinguish between identifier escape sequences (which can only include unicode escapes) and string escape sequences (which include hex/unicode/single character escapes).

@rwaldron my brain might not be working right now so a second look would be good