Support string aliases for imports/exports

[Shraymonks]

Adds support for string aliases for imports and exports.

References:
Imports
Exports

Checklist:

• All tests pass in CI.
• The script/parse-examples passes without issues.
• There are sufficient tests for the new fix/feature.
• Grammar rules have not been renamed unless absolutely necessary.
• 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).

[maxbrunsfeld]

Thanks!