[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.

goalc/make/MakeSystem.cpp

@@ -105,6 +105,7 @@ MakeSystem::MakeSystem(const std::optional<REPL::Config> repl_config, const std:
   add_tool<BuildLevelTool>();
   add_tool<BuildLevel2Tool>();
   add_tool<BuildLevel3Tool>();
+  add_tool<BuildActorTool>();
 }
 
 /*!