Add support for triple-quoted raw strings in Dart lexer

[modulovalue]

Summary

Dart supports multi-line raw strings using triple quotes:

• r'''...'''
• r"""..."""

The existing rules only handled single-quoted raw strings (r'...' and r"..."), causing triple-quoted variants to be incorrectly tokenized.

The problem

When Rouge encounters r'''...''', the existing rule r'[^']*' matches r'' as an empty raw string, leaving '...' to be parsed incorrectly as a regular string.

The fix

Add rules for triple-quoted raw strings before the single-quoted rules. Order matters because Rouge matches the first matching rule, and we need to match the longer triple-quote pattern before the shorter one.

rule %r/r""".*?"""/m, Str::Other
rule %r/r'''.*?'''/m, Str::Other
rule %r/r"[^"]*"/, Str::Other
rule %r/r'[^']*'/, Str::Other

Test case

var pattern = r'''[<>"']''';  // Was broken, now works