feat: Introduce bevy_asset_loader

[lgrossi]

Why?

Asset loading is a complex matter. Because it happens async via bevy assert server, we need to have good control over the availability of the loaded assets. bevy_asset_loader simplifies that for us, providing multiple features when it comes to asset loading: dynamic loading, folder loading, macros, etc.

What?

Currently we have a few types of assets being used in Compass:

• .dat appearances
• .json catalog
• .content.toml config
• .png images/sprites (in our case, most of them are atlases)

While the first three are trivial and can be easily ported to assets loader crate, the images are a bit more convoluted. To reduce the costs of dealing with thousand of sprites, we use sprite atlases. Although asset loader deals well with atlases, it does increase the loading time and the overall memory usage.

To balance that, we are adding sprites pre-loading as optional behind a feature, while keeping our lazy-loaded sprites as the default.