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/dashed_line.py

@@ -0,0 +1,40 @@
+"""!
+@file dashed_line.py
+@brief Turtle Challenge 2: draw a dashed line.
+"""
+
+from turtle_challenges.constants import DASH_COUNT, DASH_LENGTH, GAP_LENGTH
+from turtle_challenges.pen import Pen
+
+
+def draw_dashed_line(
+    pen: Pen,
+    dash_count: int = DASH_COUNT,
+    dash_length: float = DASH_LENGTH,
+    gap_length: float = GAP_LENGTH,
+) -> None:
+    """!
+    @brief Draw a line of dashes: pen down for a dash, pen up for a gap.
+
+    The pen is down again when the function returns, so that the caller can
+    keep drawing.
+
+    @param pen The turtle that draws.
+    @param dash_count How many dashes to draw.
+    @param dash_length Length of one dash, in turtle units.
+    @param gap_length Length of the gap after each dash, in turtle units.
+    @exception ValueError If @p dash_count is negative, or a length is not
+        positive.
+    """
+    if dash_count < 0:
+        raise ValueError(f"dash_count must not be negative, got {dash_count}")
+    if dash_length <= 0 or gap_length <= 0:
+        raise ValueError(
+            f"lengths must be positive, got dash {dash_length} and gap {gap_length}"
+        )
+    for _ in range(dash_count):
+        pen.pendown()
+        pen.forward(dash_length)
+        pen.penup()
+        pen.forward(gap_length)
+    pen.pendown()