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

tests/test_constants.py

@@ -0,0 +1,5 @@
+from turtle_challenges import constants
+
+
+def test_challenge_names_are_unique() -> None:
+    assert len(set(constants.CHALLENGE_NAMES)) == len(constants.CHALLENGE_NAMES)