CI: Determine if source files were changed before building

[Repiteo]

Implements specialization logic on the new changed-files action. This is already utilized by the godot-cpp repository to determine what source files changed, which is what I drew inspiration from. However, our repo is a fair bit more complex, so the sources needed to be expanded to cover our bases. This will mean that files only changing documentation or files that have negligible buildsystem impact will not result in GitHub Action resources. The two exceptions to this that will build regardless of changed files are:

• A run was initiated by workflow_dispatch
• If ref_name is equal to the default branch's name (aka: a push event to the main branch)

[Calinou]

Code looks good to me.

[AThousandShips]

I think this is a good improvement, it does mean that we have to keep an eye on pure documentation changes to make sure they don't do invalid changes like changing the order (and I think using some invalid syntax like & instead of &, I don't think that's enforced by the xml check), as we only get checks for that in when doing the check for class reference

It's a minor aspect but just good to make sure to track, as it's not possible to do workflow dispatch on PRs

[Repiteo]

Hmm, I hadn't considered invalid changes in documentation. It'd be nice if that could somehow be integrated as part of the xml checker, because having to build the entire engine to account for that would be rough.

[AThousandShips]

It'd be impossible to do the full checks without the editor, for example someone just adding an invalid entry, that would be trivial to detect though

Sorting entries would be more trivial to detect, but not sure how easy it would be to implement, also don't know if the syntax part is covered by the static check

To clarify though: I think this is still a worthwhile change

[Repiteo]

Well sorting wouldn't be a concern in this context, would it? It's not like a user would be adding an entirely new section if there aren't any associated code changes

[AThousandShips]

It's unlikely yes, and should be easy to discover manually

Edit: We also lose the ability in such cases to notice invalid changes to default values, so just something to keep an eye on, like a few invalid PRs in the past