Add COMPILED flag to cljs eval to turn off namespace already declared errors.

Doing this per whole eval file since the user can overwrite COMPILED whenever
they re-eval their browser.
Move cljs on-eval to its own behavior

Had to create an editor.cljs.eval tag since only editor.clj.eval
existed. Also updated cljs eval to reset COMPILED to its old
value in order to not cause any stateful issues around users
depending on COMPILED
Merge branch 'cljs-no-ns-eval'
Add a section on clojure script workflows