Report exclusion reasons in backup verbose output

Output of restic version

Source checked on master at ba802d42b7294c98b62c16d1157ea3e80820c019.

What should restic do differently? Which functionality do you think we should add?

Following #21887, I'd like to add reason categories to excluded-item output, such as pattern, marker file or filesystem boundary.

What are you trying to do? What problem would this solve?

This would help explain why a path was skipped when several exclusion rules are enabled.

Would this be a welcome follow-up? I can implement it.