games: Add game "Skull"

I'm writing my Bachelors Thesis on using CFR variants to learn Skull, and would like to add my implementation.
Note: It is a commercial game (released 2011)

[Wikipedia](https://en.wikipedia.org/wiki/Skull_(card_game))
[16min Example YouTube Video](https://www.youtube.com/watch?v=rK2locyo-Pc)

It's an elegant multiplayer (>2) imperfect information game with bidding & bluffing, so it is in a similar space to Liar's Dice, but larger and with more 'agency' around what is being bluffed rather than just responding to the roll of dice.

There's already been some academic work looking at the game:
Using DQN: `Playing the Game of Skull - a Multi-Agent Deep Reinforcement Learning approach` By Maria Kapusheva
Analyzing game-size: `A bounds-driven analysis of “Skull and Roses” cards game` By Gragera Aguaza, Baffier, Suppakitpaisarn

The issue names no files, tests, or entry points. Start by clarifying the proposed implementation and repository integration requirements for Skull; done means a reviewed, working Skull game contribution with the validation maintainers require.

Written by the indexing model from the issue text.