Add a "quick play" option that picks a random game #24

Context

Sometimes you just want to play something. A "surprise me" option is a small
feature that shows off the registry pattern nicely.

What to change

File: arcade/main.py

Add a menu option that picks a random game from GAMES and plays it.

The menu numbering is currently computed from len(GAMES) in three places
(quit_option, scores_option, and the printing in show_menu()). Adding a
fourth option by hand will make that worse. Restructure it so the extra
options are defined once, not recomputed in each function. That refactor is
the real content of this issue.

Acceptance criteria

• A "Surprise me" option plays a random game
• It announces which game was chosen before starting
• The score is saved against the correct game name
• Menu option numbers are defined in one place, not recomputed
• Adding a game to GAMES still shifts the numbers correctly
• python3 -m pytest passes

How to verify locally

python3 -m arcade     # use Surprise me a few times
python3 -m pytest

Difficulty

Intermediate. The feature is easy; the tidy-up is the point.

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