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 works, but as you can see from the TODO, we want to properly handle all of the escape sequences which are available for use in template literals. This includes all of the single-character escapes (`\r\n\`\v\f\prettymuchanything`) as well as `\uXXXX` or `\u{X...}`, or `\xXX`. There's a lot of code to be shared with string literal scanning, so it would be more useful to just plug into that.

It's been on the TODO list for a while. Want to rewrite this to A) share some existing code, and B) cover more escape sequence cases?