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

src/turtle_challenges/__init__.py

@@ -0,0 +1,7 @@
+"""!
+@file __init__.py
+@brief Solutions to the turtle graphics challenges of day 18 of Udemy's
+100 Days of Code.
+
+The challenges are added one milestone at a time; see the README.
+"""

src/turtle_challenges/constants.py

@@ -0,0 +1,15 @@
+"""!
+@file constants.py
+@brief Every constant of the project, in one place.
+
+Numbers and names used by the challenges live here and nowhere else, so a
+challenge can be tuned (for example the number of dashes) without touching its
+drawing code.
+"""
+
+## Title of the turtle window.
+WINDOW_TITLE = "Turtle Challenges"
+
+## Largest value of one color channel; also the turtle color mode, so that
+## colors can be given as RGB tuples from 0 to 255.
+COLOR_MODE = 255