Adds support for fire_atlas-based sprite sheets inside Flame.

Flame Fire Atlas is a texture atlas lib for Flame. Atlases can be created using the
Fire Atlas Editor.

Add the flame_fire_atlas dependency on your pubspec.yaml.

Then, load the atlas from your assets:

// file at assets/atlas.fa
final atlas = await FireAtlas.loadAsset('assets/atlas.fa');

or when inside a game instance, the loadFireAtlas can be used:

// file at assets/atlas.fa
final atlas = await loadFireAtlas('assets/atlas.fa');

With the instance loaded you can now get sprites and animations like:

atlas.getAnimation('animation_name');
atlas.getSprite('sprite_name');