decompiler: support merc model replacements and adding custom actor models to vanilla fr3s

[Hat-Kid]

This adds support for replacing existing merc models in FR3 files with custom GLB model files. The replacements go in custom_assets/<GAME>/merc_replacements, similar to texture replacements. When a .glb file with a file name that matches any model present in an FR3 is detected (e.g. eichar-lod0 for Jak), all merc model data is replaced with the given model.

Additionally, models for custom actors can now also be added to vanilla FR3s. The models for this go in custom_assets/<GAME>/models/<LEVEL_NAME> (e.g. custom_assets/jak1/models/jungleb/test-actor-lod0.glb) and will be added to the FR3 that has a matching name (exception: to add things to the common level file, the folder should be named common instead of GAME).
For custom levels, these now go in custom_assets/<GAME>/models/custom_levels (previously custom_assets/<GAME>/models).

Another small change: When level ripping is enabled, the resulting model files will now be stored in game name subfolders inside of glb_out.