feat: allow enums to use double quoted strings and here/nowdocs

[calebdw]

Checklist

• All tests pass in CI
• There are enough tests for the new fix/feature
• Grammar rules have not been renamed unless absolutely necessary (x rules renamed)
• The conflicts section hasn't grown too much (x new conflicts)
• The parser size hasn't grown too much (master: STATE_COUNT, PR: STATE_COUNT)
  (check the value of STATE_COUNT in src/parser.c)

Fixes #195 (note this was branched off of #192).

Apparently using heredocs/nowdocs in an enum is valid PHP:

enum Suit: string
{
  case Hearts = 'H';
  case Diamonds;
  case Clubs = "C";
  case Spades = <<<TXT
      S
      TXT;
}

[cfroystad]

Nice work! This looks correct. I'll have another look and merge after #192 has landed