Add regular expression s flag highlighting.

[CentricStorm]

Description of the Change

1. Adds support for regular expression s(dotAll) flag syntax highlighting, as the ECMAScript proposal has reached stage 4, and is considered a "finished proposal".
2. Adds a third scope - meta.flag.regexp - to all regular expression flags (no change to existing scopes).
3. Adds unit tests for both single- and multi-flag regular expression syntax highlighting.

Alternate Designs

The meta.flag.regexp scope was chosen instead of meta.flag.regexp.js to be consistent with other regular expression scopes like meta.group.assertion.regexp.

Benefits

Adds support for the regular expression s(dotAll) flag, and enables custom syntax highlighting of all regular expression flags.

Possible Drawbacks

There weren't previously any unit tests testing regular expression flag highlighting, so there weren't any to test against.

Applicable Issues

None.