Add a step in CI to check if generated files (like `grammar.json`) are updated during PRs

[Jeremy Goh]

As per title.

As of now, we are still required to check in tree-sitter generated files into the git repository. So whenever we make changes to grammar.js, we need to push the changes to the generated files along with it.

Suggest to add a check during CI to enforce we don't forget to do this. Probably some version of tree-sitter generate and git status --porcelain / git diff should do the trick.

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I can take this up