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

pyproject.toml

@@ -25,6 +25,9 @@ dev = [
     "ruff>=0.8",
 ]
 
+[project.scripts]
+turtle-challenges = "turtle_challenges.cli:main"
+
 [tool.setuptools.packages.find]
 where = ["src"]