Enhancement: Introduce REUSE for the license declaration of code (and other) files

Since it appears that we are increasingly using REUSE to declare the licensing of image files (see #2706), I think we should use REUSE to do the same for files that contain code too. Thoughts?

Review the REUSE approach referenced in this issue and compare it with the image-file work in #2706. No files or tests are named; identify the repository's code and other applicable files, then confirm that their licensing declarations follow the chosen REUSE convention.

Written by the indexing model from the issue text.