Add `await` as reserved identifier

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Checklist:

• [ ] All tests pass in CI.
• [ ] 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.
• [ ] The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

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Needs further testing