[wip] build actor tool (#3266)

This does a couple of things:

- The `custom_levels` folder was renamed to `custom_assets` and contains
`levels`, `models` and `texture_replacements` folders for Jak 1, 2 and 3
in order to keep everything regarding custom stuff in one place.
- With this, texture replacements now use separate folders for all games
- A build actor tool was added that generates art groups for custom
actors
- Custom levels can now specify what custom models from the `models`
folder they want to import, this will add them to the level's FR3.
- A `test-zone-obs.gc` file was added, containing a `test-actor` process
that uses a custom model as an example.

The build actor tool is still very WIP, the joints and the default
animation are hardcoded, but it allows for importing any GLB file as a
merc model.

decompiler/main.cpp

@@ -322,7 +322,8 @@ int main(int argc, char** argv) {
     tex_db.merge_textures(texture_merge_path);
   }
 
-  auto replacements_path = file_util::get_jak_project_dir() / "texture_replacements";
+  auto replacements_path = file_util::get_jak_project_dir() / "custom_assets" /
+                           game_version_names[config.game_version] / "texture_replacements";
   if (fs::exists(replacements_path)) {
     tex_db.replace_textures(replacements_path);
   }