Add action to check if CHANGELOG.md was updated

[crnh]

Proposed change

We often forget to update the changelog. This action checks a PR for an updated changelog (it's output should be ignored for PRs that don't require changelog updated).

Type of change

• [ ] Example (a notebook demonstrating how to use ZOSPy for a specific application)
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New analysis (a wrapper around an OpticStudio analysis)
• [ ] New feature (other than an analysis)
• [ ] Breaking change (fix/feature causing existing functionality to break)
• [x] Code quality improvements to existing code or addition of tests

Additional information

• Python version: N/A
• OpticStudio version: N/A

Related issues

Checklist

• [x] I have followed the contribution guidelines
• [ ] The code has been linted, formatted and tested locally using tox.
• [ ] Local tests pass. Please fix any problems before opening a PR. If this is not possible, specify what doesn't work and why you can't fix it.
• [ ] I added new tests for any features contributed, or updated existing tests.
• [ ] I updated CHANGELOG.md with my changes (except for refactorings and changes in the documentation).

If you contributed an analysis:

• [ ] I did not use AttrDicts for the analysis result data (please use dataclasses instead).

If you contributed an example:

• [ ] I contributed my example as a Jupyter notebook.

[crnh]

I've been unable to test the "skip changelog" label (which is intended to disable this check) but can test it later if you want

[crnh]

It has now been tested in #33!