Make project REUSE-compliant

[robo9k]

REUSE is a way to add machine-readable copyright and licensing information to a project.

This merge requests makes this project REUSE-compliant. It does not change the license or copyright of anything, it just adds metadata. The license is still MIT (but see #12 and the existing license section in README).
LicenseRef-file is from file/fileCOPYING

As a further step, the README should probably contain a "contribution" section stating how contributions will be licensed.
This merge request also adds a CI workflow that should fail if any files without REUSE metadata are found.

[robo9k]

$ askalono id LICENSES/LicenseRef-file.txt
License: BSD-2-Clause (original text)
Score: 0.842