Extend export syntax, piggybacking on javascript grammar

[mjambon]

This is the continuation of #125. It uses the latest tree-sitter-javascript which now supports extended export syntax for javascript.

Checklist:

• All tests pass in CI.
• 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).