Add template workflow to check the license file

[per1234]

Whenever one of the recognized license file names are modified in the repository, the workflow runs licensee to check whether the license can be recognized and whether it is of the expected type.

GitHub has a useful automated license detection system that determines the license type used by a repository, and surfaces that information in the repository home page, the search web interface, and the GitHub API. This license detection system requires that the license be defined by a dedicated file with one of several standardized filenames and paths.

GitHub's license detection system uses the popular licensee tool, so this file also serves to define the license type for any other usages of licensee, as well as to human readers of the file.

For this reason, and to ensure it remains a valid legal instrument, it's important that there be no non-standard modifications to the license file or collisions with other supported licence files. This workflow ensures that any changes which would change the license type or which license file is used by the detection are caught automatically.

Notes

• I chose not to take the usual task-based approach for this one because I think the value of that comes from facilitating running common development processes locally. But I don't think this is common, nor something anyone would bother running. Instead, it's something that can be added to the repository and forgotten until the day comes that someone mucks around with the license file.
• Although this workflow is in use in this repo, it is not possible to set up a sync because the CI workflow was modified from the template to use the CC0 license.