feat(multiplayer): Add character selection system

[pevides]

This PR refactors the local multiplayer player initialization to remove hardcoded paths to Tux's sprites, introducing a dynamic Character Profile system. Players (in either multiplayer or singleplayer) can now select between Tux, Penny, and Larry in the multiplayer menu using the arrow keys.

Since full animation sheets for Penny and Larry do not exist yet, we implemented static placeholder sprites to make the feature testable.

This PR also lays the groundwork for future gameplay diversification. By decoupling the player logic from hardcoded assets, the game can now easily be expanded to support characters with unique stats, or new abilities, or gameplay mechanics in the future.

Video Showcase: https://www.youtube.com/watch?v=kMhap_W9TzM

[swagtoy]

Quick skim; probably won't be a while until I (or someone else) reviews this due to its scale and priorities, but: I feel like we should move the list of characters into a separate s-expr file instead of hardcoding it directly in some places.

Something like

(supertux-playerlist
  (player tux "spriteinfo.sprite" blahblah)
  (player penny "penny.sprite" blahblah)
  (player larry "larry.sprite" blahblah)
)

I would probably go ahead and try to have a stab at doing that.

Also, we don't do feat(XXX) style commits, but it doesn't matter since we squash and rebase anyway and we can change the name there.

[pevides]

Thanks for the feedback! I've moved the character list out of C++ into a new data/characters.stlst s-expr file using a (supertux-playerlist ...) root element. CharacterRegistry now parses it at startup via ReaderDocument, following the same pattern used in addon_manager.cpp. Is this what you meant me to do?