Add Continuous Integration

[DanWaxman]

This PR does a few things:

• Add continuous integration, including linting and testing
  • Note: I didn't want to undertake significant refactors in dynamax, so these are ignored.
• Merges the dev_numpyro_api and public branches
• Makes many small edits for linting
  • Warning: these are mostly LLM-generated refactors, as the workload would be enormous otherwise, but almost always for very simple and explicitly specified issues (e.g., "Resolve the linting errors outputted in lint_output.txt, including: [E722] use the correct exception (usually an AssertionError), [E402] move imports to the top where needed, [F821] add imports where missing").
  • I've checked all by hand, but please do double-check these.

Remaining to-do:

• Refactor tests to use pytest, be manageable on a CI machine, and make sure those pass.
• Consider doing type checking with mypy. This might be very high-effort though.

In general, one may now use make clean to run relevant ruff format and ruff check --fix commands (excluding notebooks and dynamax), make lint to lint, and make test to test.

[DanWaxman]

Changed the base branch to dev-numpyro-api after discussion: it seems there might be some additional work for that branch to be merged to public. In any case, this now syncs dev-numpyro-api to the changes made in public.