File preprocessing

[thomas-zahner]

Document the new preprocessor feature.
Ideally, we should only merge this with the next lychee release to prevent confusion by users.

[thomas-zahner]

I've now worked around the regex size error that occurs on all new PRs. In the process I greatly simplified the link checking. (see last 3 commits) I'm now simply checking all links instead of only the new ones. As we seem to use a pretty fast link checker this seems to be no problem and the step completes in ~5 seconds. 🙃

So I see no good reason to stick to this overly complicated approach of only checking additions.Also I noticed that the src directory wasn't checked previously. In total the workflow should be faster because we have much fewer steps and we don't build twice.

[mre]

Wow, this is really high quality work. You managed to significantly simplify the workflow. Good job! Also, thanks for following the boyscout rule and cleaning up the ignore file.

[mre]

As a side note, I'm amazed at how fast lychee is. 5 seconds for the link check is amazing. Most of it is thanks to Rust and its ecosystem, of course. And fast servers.