feat: support $$ escape in string interpolation

[yinho999]

This PR added $$ as an escape sequence inside backtick-string interpolation that emits a literal $.
For example, $${...} produces a literal ${...} instead of triggering interpolation.

Fixes #694

[schungx]

I would rather not make $$ special... instead, I've put together another version #1089 that uses \${...} to escape the ${...}.

This is consistent with JavaScript.

Standalone \$ does not do anything and still yields \$. Therefore it requires three characters in a row \${ to trigger, and unget one if the third one is not {.

I have moved your tests to the new version as well.