Move base64-encoded data out of .tscn files

[akien-mga]

Before:

$ ls -lh Box/DestroyedBox.tscn Environment/environment.tscn Player/GrenadeVisuals/grenade/grenade.tscn 
-rw-r--r-- 1 akien akien  13M Aug 15 17:15 Box/DestroyedBox.tscn
-rw-r--r-- 1 akien akien 949K Aug 15 17:15 Environment/environment.tscn
-rw-r--r-- 1 akien akien 5,3M Aug 15 17:15 Player/GrenadeVisuals/grenade/grenade.tscn

After:

$ ls -lh Box/DestroyedBox.tscn Environment/environment.tscn Player/GrenadeVisuals/grenade/grenade.tscn 
-rw-r--r-- 1 akien akien 8,8K Aug 15 17:15 Box/DestroyedBox.tscn
-rw-r--r-- 1 akien akien 798K Aug 15 17:15 Environment/environment.tscn
-rw-r--r-- 1 akien akien 1,5K Aug 15 17:15 Player/GrenadeVisuals/grenade/grenade.tscn

environment.tscn still has some collision shapes embedded that could be extracted.

Draft as there's still a major issue with this, some of the newly created .res files change every single time the project is saved... The destroyed box crate stuff is also weirdly heavy and hacky, ideally we should find a way to source those meshes from the .glb file.

[akien-mga]

Superseded by #30.