Add the command line entry point

python -m turtle_challenges <challenge> and the turtle-challenges command open a
window, run the named challenge and wait for a click. square and dashed-line are
registered; a missing Tk is reported with a hint instead of a traceback.

Task: MIL-002#6

src/turtle_challenges/__main__.py

@@ -0,0 +1,9 @@
+"""!
+@file __main__.py
+@brief Entry point of `python -m turtle_challenges`.
+"""
+
+from turtle_challenges.cli import main
+
+if __name__ == "__main__":
+    raise SystemExit(main())