Better handling of generated files

[resolritter]

tree-sitter generate updates the parser's files after grammar changes. Tree-sitter grammars usually adopt one of the following strategies for dealing with those files:

1 - Commit the changes to the main branch through PRs, as done in e.g. tree-sitter-typescript; see https://github.com/tree-sitter/tree-sitter-typescript/pull/241/files for example
2 - Commit the changes to a release branch, as done in e.g. tree-sitter-swift; see alex-pinkus/tree-sitter-swift#149 (comment) and https://github.com/alex-pinkus/tree-sitter-swift/tree/with-generated-files for example

I infer that this repository wants to abide by strategy 1. If so, a CI check should be added to ensure that the parser's files are up-to-date when a PR is submitted. The lack of such check is detrimental to the grammar because, otherwise, reviewers have to remember to ask the PR submitters to update and commit the files before merge; as an aside, the reminder did come up when I submitted #26 and as a result the parser's files in main are currently outdated. That could have been prevented with a CI check along those lines:

make generate
git diff-index --quiet HEAD || \
  { >&2 echo 'Please run `make generate` and commit the files'; exit 1; }

Please let me know if you want to implement that check on CI or do it some other way.

[resolritter]

For what it's worth, I think strategy 2 is cleaner wrt. the commit history. In #33 I went with strategy 1 because it's more in line with what's already done in this repository.