Add Turtle screen, game loop and entry point (MIL-003)

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4
Task: MIL-003#5
Task: MIL-003#6

src/us_states_game/__main__.py

@@ -0,0 +1,9 @@
+"""Runs the game with `python -m us_states_game`.
+
+@file __main__.py
+@brief Module entry point that calls main().
+"""
+
+from us_states_game.main import main
+
+raise SystemExit(main())