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/constants.py

@@ -13,3 +13,21 @@ 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
+
+## Degrees in a right angle.
+RIGHT_ANGLE_DEGREES = 90
+
+## Challenge 1: number of sides of a square.
+SQUARE_SIDES = 4
+
+## Challenge 1: length of one side of the square, in turtle units.
+SQUARE_SIDE_LENGTH = 100
+
+## Challenge 2: number of dashes in the dashed line.
+DASH_COUNT = 50
+
+## Challenge 2: length of one dash, in turtle units.
+DASH_LENGTH = 10
+
+## Challenge 2: length of the gap after each dash, in turtle units.
+GAP_LENGTH = 10