Add debug or trace output when files with unsupported extensions are found

When you run `copywrite` on files with unsupported extensions, you get no information that these files are skipped.

Getting a debug output of why the file was skipped would be nice.

The only other way to find out is to look at the source code.

It was discussed during a review: https://github.com/hashicorp/copywrite/pull/85#discussion_r1306013851