Watches lib/ for file changes and triggers recompilation + re-render.

Only starts in :dev environment, and not when config :raxol, :code_reloader is false. Uses the file_system library to watch for *.ex changes, debounces rapid edits, then calls IEx.Helpers.recompile/0 and sends :render_needed to the Lifecycle. The recompile prints Mix's output to stdout.