Add regular expression lookbehind highlighting.

[CentricStorm]

Description of the Change

1. Added support for regular expression lookbehind assertion syntax highlighting, as the ECMAScript proposal has reached stage 4, and is considered a 'finished proposal'.
2. Added unit tests for both lookahead and lookbehind syntax highlighting.

Alternate Designs

The meta.assertion.look-behind.regexp scope was chosen so as to be consistent with the existing meta.assertion.look-ahead.regexp scope, though technically meta.assertion.lookbehind.regexp would have been better, as the term 'lookahead' (no hyphen) is used in the ECMAScript language specification.

Benefits

Enables custom syntax highlighting of regular expression lookbehind assertions.

Possible Drawbacks

There weren't previously any unit tests testing regular expression lookahead/lookbehind assertion highlighting, so there weren't any to test against.

Applicable Issues

None.

[winstliu]

the ECMAScript proposal has reached stage 4, and is considered a 'finished proposal'.

🎉

Added unit tests for both lookahead and lookbehind syntax highlighting

😍