Adding clj-kondo as the first action

Execute tests via GitHub actions since TravisCI is being retired

Have separate configs for linting & for tests to keep things simple

Since they run in two separate file, it produces two actions with the
same name, that of the subject of the commit. Which is confusing, so
joined the jobs in one YAML file because that presents a cleaner
overview in GitHub Actions web UI, as you can see the names of all the
jobs and the outcome.

Added deps.edn because it's about time to migrate to it.

Added GitHub Action for running the tests via Clojure CLI, so that I
can smoothly migrate off of project.clj/Leiningen.

Moved the CI functionality to GitHub Actions because TravisCI is
retiring its CI functionality for open source projects for free.

I don't want the link on the badge to take you to the badge, but to
the outcome of GitHub Action.

Since https://versions.deps.co/ doesn't work reliably, use this,
"manual", way of verifying dependencies.

Test across as many Java versions for both Clojure CLI & Leiningen
based setups

The cron line looks not indented here:

https://github.com/marketplace/actions/scheduled-issue