fix: allow digits in heredoc identifier

[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 (0 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)

Hello! This PR closes #209 and allows digits in heredoc identifiers, which is valid PHP.

Thanks!

[calebdw]

Thanks @amaanq!