JS: Better regex detection

[RunDevelopment]

This fixes some of the issues addressed in #2154.

The changes I made can be summed up as:

1. Regexes can now be followed by a :.
2. There can be any number of spaces and comments between the regex and what comes after. But only multiline comments because single-line comments are always followed by either a newline or the EOF which both are accepting.

I just want to point out, that I cannot add this comment or whitespace regex to the lookbehind because JS comment cannot be parsed in RTL direction.