fix: single types being incorrectly parsed as a union_type

[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!

I noticed that single types were being incorrectly parsed as a union_type, this PR fixes that issue so they are correctly parsed as a singular type (note this was branched from #192):

<?php
function a(): int {}

---

(program
  (php_tag)
  (function_definition
    (name) (formal_parameters)
-    (union_type (primitive_type))
+    (primitive_type)
    (compound_statement))

Thanks!

[calebdw]

See #214