Add draw_square and draw_dashed_line with tests

Challenge 1 draws four sides and turns right 90 degrees after each. Challenge 2 puts
the pen down for a dash and up for a gap and leaves it down at the end. Lengths and
counts come from constants.py; invalid values raise ValueError.

Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5

src/turtle_challenges/__init__.py

@@ -3,5 +3,15 @@
 @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.
+The challenges so far are @ref turtle_challenges.square.draw_square and
+@ref turtle_challenges.dashed_line.draw_dashed_line. Each takes the turtle as its
+first argument, so it can be tested without a window.
 """
+
+from turtle_challenges.dashed_line import draw_dashed_line
+from turtle_challenges.square import draw_square
+
+__all__ = [
+    "draw_dashed_line",
+    "draw_square",
+]