Adopt constants.py and its test

Copied from the base with identity-only edits: the docstrings name days 20 and 21, and the package main page describes the whole game. The values are unchanged; MIL-003 checks them against the day-21 lectures.

Task: MIL-001#3
Closes #3

src/snake_game/__init__.py

@@ -0,0 +1,13 @@
+## @package snake_game
+#  @brief Snake game from Udemy's 100 Days of Code (day 21).
+#
+#  @mainpage Snake Game
+#
+#  A snake of three square segments moves across a Python turtle screen by
+#  itself and is steered with the arrow keys. It eats food, grows and keeps a
+#  score, and the game is over when the head passes the wall or touches the
+#  tail. The game uses the standard library only.
+#
+#  The constants of the game are in constants.py.
+
+"""Snake game from Udemy's 100 Days of Code (day 21)."""