Make babashka compatible

This PR makes markdown-clj babashka compatible:

Currenty SCI doesn't understand the metadata in declare so I defined the dynamic vars using def which has no impact on the implementation.
Added a deps.edn so this repo can be used as a git dependency.

To run babashkabb test:bb to run tests in babashka. This task is defined in bb.edn.

While I was at it, I also added a test runner for Clojure and Clojurescript in bb.edn:

bb test:clj
bb test:cljs

yogthos / markdown-clj