Fix processed RON assets being ignored (#48)

## Summary
- share custom RON asset loaders between the game and asset processor
- register image, font, and glTF loaders in the headless processor
- prevent `game_settings.ron` and `game.stylesheet.ron` from receiving ignored processed metadata

Reviewed-on: #48
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/menus/mod.rs

@@ -6,12 +6,10 @@ use bevy::{
     prelude::*,
 };
 
-use ron_asset_manager::RonAssetPlugin;
-
 use crate::{
     AppState,
     game_settings::CommonAssets,
-    menus::{character_select::CharacterSelectScreen, main_menu::MainMenu, style::FGUiStyle},
+    menus::{character_select::CharacterSelectScreen, main_menu::MainMenu},
 };
 pub mod button;
 pub mod character_select;
@@ -222,7 +220,6 @@ impl Plugin for MenuPlugin {
         //---------
         // STAGE SELECT END
         //---------
-        .add_plugins(RonAssetPlugin::<FGUiStyle>::create("stylesheet.ron"))
         .add_systems(
             Update,
             (