Update scrimage to 4.x from 2.x & fix copying resources

[sloshy]

Scrimage, a dependency of sbt-microsites, has actually been significantly out of date for some time, probably due to a dependency name change from scrimage-core to scrimage-scala for the Scala-specific dependencies. This updates the library, and replaces the one usage of it using the latest syntax.

EDIT: The scope of this has expanded slightly to also fixing the build, by rewriting the copyJARResourcesTo function (now copyResourcesFromFileSystem) to work with arbitrary filesystems. While I'm not entirely sure what the cause of the build issues was, this issue might have had something to do with the specific JDK I am using (adoptopenjdk 8) as the issue was nonexistent before, and only popped up recently in the build.

[BenFradet]

👍

[sloshy]

Currently working with the scripted tests, as these are failing now. The new copying resources code isn't quite matching up and I've got a lead on where to fix it at least. Hold off on reviewing for now.

[sloshy]

The build is "fixed" in terms of, if you pull this branch locally and run sbt ci-test, it will work assuming you have jekyll set up. Apparently there's a problem with the jekyll setup on GitHub Actions, so I've logged that in a comment on #531. For all intents and purposes I think this PR is technically reviewable again. Please run the tests locally if you want to confirm that it's just github having a jekyll problem.