This adds 2 mypy GitHub Actions checks:

• mypy.yml runs mypy on every PR. This will list all errors, so it shouldn't block until errors are resolved.
• mypy-new-errors.yml compares the PR branch to main to ensure no new mypy errors are introduced. This check can be made blocking now and can be removed once all errors are resolved in favor of mypy.yml.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.