Set up GitHub actions to run test automation on push to branch and merge to master

[benfowler]

Random breakages due to upstream breaking changes has been a thing for a little while now. It would be nice to catch these early.

Prior art:

See examples in other Telescope plugins listed here. Here are some other Telescope extensions which define GitHub action that executes Lua unit tests:

• cljoly/telescope-repo.nvim: Makefile-driven Lua test runner with minimum reproduction of the environment
• dhruvmanila/telescope-bookmarks.nvim: GitHub actions, launching Lua+Vusted for tests; Styla for lint warnings

Bonus credit: run Stylua to lint the source code

Depends on #5 .