Add project configuration, .gitignore and the package skeleton

pyproject.toml declares Python 3.13 or later, an empty runtime dependency list and
pytest, ruff and mypy as development tools. The package has constants.py, the one
place for constants, and a smoke test keeps the test run green.

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3

tests/test_package.py

@@ -0,0 +1,14 @@
+import turtle_challenges
+from turtle_challenges import constants
+
+
+def test_package_can_be_imported_and_is_documented() -> None:
+    assert turtle_challenges.__doc__
+
+
+def test_color_mode_is_the_rgb_range_of_the_course() -> None:
+    assert constants.COLOR_MODE == 255
+
+
+def test_window_title_is_not_empty() -> None:
+    assert constants.WINDOW_TITLE