• Update to Bevy 0.6
• Update Rust edition to 2021 to follow Bevy
• Remove WebGL2 renderer; Bevy 0.6 handles wasm directly
• Remove custom shader for Boot to simplify it and avoid having to write an entire custom pipeline just to animate a progress bar

Add an AppState::Boot state before the main menu, which loads the critical assets (mainly, fonts) required to display a loading screen, and without which the game cannot start. The sequence should be kept minimal as no user interaction can be provided during this time. Most loading should occur during the AppState::MainMenu state that follows, where some progress and feedback can be displayed.

Refactor the game code as more ECS-oriented, partitioning it in more coherent systems for the inventory management, the level loading, and the deserialization of the levels from disk.

Move levels definition to assets/levels.json, and load it via a custom TextAsset asset. This ensures it can be loaded in wasm. Handle async loading with "Loading..." text on main menu, and extra MainMenuLoading app state.

• Add cargo make targets
• Add web and native features
• Switch rendering to webgl2 for wasm, as wgpu is not supported in browsers
• Convert sound to OGG, as bevy_kira_audio does not support MP3 on wasm
• Add controls on web page directly for ease of use
• Work around AssetFolder::load_folder() not implemented in wasm (bevyengine/bevy#2916)