This is the final PR related to #4408

This PR includes 2 changes:

• run the code formatter automatically after code generation. This should make it more immediately obvious what changed during codegen without the mandatory manual step of running the formatter
• add a github action which runs the code generators and formatter and fails if the diff from this is non-empty. This will ensure that nothing in the PR will be overwritten by code generators later on and that everything which needs to be generated was (avoiding a situation where contributors manually implement "generated" methods)

I tested this in johannescoetzee#1. Some of the later commits are missing since I cleaned up the branch for this PR, but the relevant github jobs were:
Working run
https://github.com/johannescoetzee/javaparser/actions/runs/9485459827/job/26137443518?pr=1

Incorrect formatting
https://github.com/johannescoetzee/javaparser/actions/runs/9485678529/job/26138198159?pr=1

Fixed again
https://github.com/johannescoetzee/javaparser/actions/runs/9485811857/job/26138677223?pr=1

Missing generated code
https://github.com/johannescoetzee/javaparser/actions/runs/9485852607/job/26138825304?pr=1

We could potentially remove the checkstyle job, but I'm not sure whether that checks anything not covered by spotless. I'll do a bit of research on that.