Improve tokenization of regular expressions

[spahnke]

Hi,

this PR does three things

1. Recognize s as a valid regular expression flag
2. Tokenize the flag part of regular expressions as keyword.other like VS Code does
3. Add more tests for regular expressions

Let me know what you think.

Thanks!

[msftclas]

All CLA requirements met.

[rebornix]

@spahnke thanks for your contribution, it looks solid and helps me know more about the newly added regex flag https://tc39.github.io/ecma262/#sec-get-regexp.prototype.flags