Fix require-unicode-regexp issues

[whymarrh]

Refs #8982

See require-unicode-regexp for more information.

This change enables require-unicode-regexp and fixes the issues raised by the rule.

But what is the u flag?

Unicode-aware regular expressions in ES2015 covers the most of it, I'd recommend skimming the sections.

My opinionated summary:

ECMAScript 2015 introduces [...]

1. [...]
2. u enables various Unicode-related features.

Impacts of the u flag:

• Things like \u{1234} are now Unicode code point escapes instead of matching 1234 consecutive u symbols
• Things like \a (where a is not an escape character) now error because \a is not a reserved escape sequence
• The . now matches unicode symbols
• Quantifiers e.g. *, +, ?, and {2}, {2,}, {2,4} now work on unicode symbols
• Character classes and negated character classes work on unicode symbols
• Combining the u and i flags is weird because it does case folding

Recommendations for developers

• Use the u flag for every regular expression you write from now on.
• …but don’t blindly add the u flag to existing regular expressions, as it might change their meaning in subtle ways.
• Avoid combining the u and i flags. It’s better to be explicit and include all letter cases in your regular expression itself than to be surprised by implicit case folding.

[Gudahtt]

LGTM! I can't see any cases of invalid escape sequences, and I can't think of any reason we'd want to avoid matching unicode characters in any of these cases.