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/bin/process-assets.rs

@@ -1,8 +1,11 @@
 use bevy::{
     app::ScheduleRunnerPlugin,
     asset::{AssetMode, AssetPlugin, processor::AssetProcessor, processor::ProcessorState},
+    gltf::GltfPlugin,
+    image::ImagePlugin,
     prelude::*,
     tasks::block_on,
+    text::TextPlugin,
 };
 use shinespark::AssetProcessingPlugin;
 use std::{process::ExitCode, time::Duration};
@@ -15,6 +18,7 @@ fn main() -> ExitCode {
             use_asset_processor_override: Some(true),
             ..default()
         })
+        .add_plugins((ImagePlugin::default(), TextPlugin, GltfPlugin::default()))
         .add_plugins(AssetProcessingPlugin)
         .add_systems(Update, finish_when_processed);