Add Champ stealth mini game

[felixwalberg]

The stealth mini game will be the final stop on Evan's journey to find Champ. It leads him out onto the Causeway (a long gravel path into the lake), where at long last, he spies the lake monster. This mini game will be home to many spots for learners to make contributions, particularly engineering ones.

This PR contains a tilemap, narratives, and some simple hint npcs that gently guide the learners. The character is also a simplified version of the Player class and will only give the learners left and right movement. It is possible to complete the level by walking to the right until you hit a teleporter. One unresolved issue with the basic character class is the behavior with checkpoints (#1885).

There is still a plan to add many more features that can be built upon by learners. This includes more commented-out functions, such as the ability to walk on water, more interactions with the guard mechanics (possibly things that swim and jump out across the pathway), and other "power-ups" such as sprinting/boost or Justin's "blink" ability.

It would be great to get this merged in (even though it lacks some of these features) so multiple people on the team can experiment with other ideas.

Assets:
shore_and_lake_tiles_and_border.png by @Fauche-r

Engineering:
Incomplete Character by @felixwalberg
NPC Hints and "broken" guards by @felixwalberg
Randomized water animation by @shadeydo

Game Design:
Progression & Motivation by @felixwalberg
Game Feel by @shadeydo

Production and Management:
Code Review by @Fauche-r @felixwalberg @shadeydo

[github-actions]

Play this branch at https://play.threadbare.game/branches/endlessm/champ-stealth-mini-game.

(This launches the game from the start, not directly at the change(s) in this pull request.)