{
        className: 'string',
        variants: [
          { begin: /b?r(#*)"(.|\n)*?"\1(?!#)/ },
          { begin: /b?'\\?(x\w{2}|u\w{4}|U\w{8}|.)'/ }
        ]
      },

I'm guessing . doesn't cover Emoji... I'd have to play around with this one...

Would using |\p{Extended_Pictographic} be acceptable?
Quick checks seem to make it work.

https://regexr.com/7o2cc

I talked a bit too fast, the pattern above does not work either (I think ✨ is encoded using only one codepoint, the problem seems to be for characters encoded with 2 codepoints).
After looking at the char implementation, the following are also valid but not matched by the expression above:

• '\u{10ffff}'
• '\u{FFFD}'
• 🥰 (actually, most unicode emojis seem to fail)
• '𝕊'

I really am not a regex expert, but I found the following matches for emojis / unicode characters, this might be of some help:

• https://www.regextester.com/106421
• https://unicode.org/reports/tr51/#EBNF_and_Regex
• https://www.npmjs.com/package/emoji-regex

Want to see if my fix works?