fix: add single quote string escape_sequence

[calebdw]

Checklist

• [xi] All tests pass in CI
• There are enough tests for the new fix/feature
• Grammar rules have not been renamed unless absolutely necessary (0 rules renamed)
• The conflicts section hasn't grown too much (0 new conflicts)
• The parser size hasn't grown too much (master: 2681, PR: 2682)
  (check the value of STATE_COUNT in src/parser.c)

Closes #173

[cfroystad]

This is great work! Thank you for all your effort on improving the parser!