Fix literal \ characters at the end of a string

Fix literal \ characters at the end of a string

Given a statement like `{% set sep = '\\' %}`, the syntax highlighter was considering the literal `'\\'` to be a literal quote inside the string by matching `\'` and thus highlighting the rest of the entire document as a string.

This change fixes that issue by adding some regex to ensure that an escaped quote is preceded by an uneven number of slashes (that is, `'\\\\\''` would accept the literal string `\\'` without issues). A`"` quoted string is modified the same way.