Automatic PR closure upon failure to adhere to template + weird checkbox whitespace fix

[Thodor12]

Checklist

• I have read and accept the Contributing guidelines
• I have tested and confirmed my changes are working on the most recent commit of this pull request
• I have used AI in the creation of this pull request

Changes proposed in this pull request

• Introduce a new reusable workflow (this is not automagically applies, needs to be first enabled in each individual repository) that checks if specific (configurable via inputs) lines are present on the body of the PR. I used this to basically enforce the top-most checklist only. As it's hard to require the rest of the content, which is dynamic, to be fixed per PR.
  Also we sometimes omit things like the related issues, so it's not worth to omit that, only verifying the checklist exists is enough, because that contains all the checkboxes.
• Offending pull requests get a comment placed on them, and are automatically closed and locked for comments. (This is both configurable, defaults to true)

Review please

[MotionlessTrain]

This will fail on “[ x]”, which is very easy to (accidentally) do whilst filling in the template

[Thodor12]

I have loosened the regexes slightly. Between the [] it may now contain any combination of xX as many times as possible.