fix: add for as a highlight keyword

[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)

Hello!

This closes #165.

Thanks!