Add hexa game

[rudifa]

This branch includes the previous branch add-demo-game. It is based on the current main branch:

arcade % tree webapp/components                                                              [main|✔]
webapp/components
├── app-game.js
├── app-start.js
└── game-button.js

I renamed app-game.js -> demo-game.js, added code from branch add-hexagon-groups and moved code into per-game subdirectories:

arcade % tree webapp/components                                                              [add-hexa-game|✔]
webapp/components
├── app-start.js
├── game-button.js
└── games
    ├── demo-game
    │   └── demo-game.js
    ├── hexa-game
    │   ├── app-pixi.js
    │   ├── buttons.js
    │   ├── hexa-game.js
    │   ├── hexagrids-and-groups.js
    │   ├── interactive-mixins.js
    │   ├── redblobgames
    │   │   ├── cli-demo.js
    │   │   └── lib-module.js
    │   ├── shapes.js
    │   ├── spare-parts
    │   │   ├── hack-board.js
    │   │   ├── pixi-application.js
    │   │   ├── pixi-buttons.js
    │   │   ├── pixi-graphics.js
    │   │   └── toggle-button.js
    │   └── sprites.js
    └── other-game