Implement coffee machine core for MIL-002

Add the menu, coin values, starting resources and message texts to
constants.py. Implement report, is_resource_sufficient, process_coins,
is_transaction_successful, make_coffee and the main loop. Money is
counted in whole cents. The smoke test now feeds "off" so the loop
exits.

Closes #6
Closes #7
Closes #8
Closes #9
Closes #10
Closes #11
Closes #12

Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5
Task: MIL-002#6
Task: MIL-002#7

tests/test_smoke.py

@@ -3,11 +3,14 @@
 @brief Smoke test that the package imports.
 """
 
+import pytest
+
 from coffee_machine import main as main_module
 
 
-def test_main_is_callable() -> None:
+def test_main_is_callable(monkeypatch: pytest.MonkeyPatch) -> None:
     """!
-    @brief The entry point runs without error.
+    @brief The entry point starts and stops on "off".
     """
+    monkeypatch.setattr("builtins.input", lambda _prompt: "off")
     main_module.main()