Properly parse 'as const'

[guillaumebrunerie]

This pull request improves how as const is parsed. Currently, an expression like "foobar" as const is parsed as if const were a type identifier, which is wrong, it is a keyword.

Checklist:

• [X] All tests pass in CI.
• [X] There are sufficient tests for the new fix/feature.
• [X] Grammar rules have not been renamed unless absolutely necessary.
• [X] The conflicts section hasn't grown too much.
• [X] The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c). (STATE_COUNT increased from 4437 to 4439)

[guillaumebrunerie]

Closing in favor of #252.