Repository: godot
Issue: Godot 4.0 GLTF Importer missing "instanced sub-scenes" storage option from 3.4 (important for asset packs etc.)
Labels: bug,topic:import,regression,topic:3d

### Godot version

v4.0.dev.calinou [71616630e]

### System information

Windows 10, Ryzen 3700x, 32GB, 3060Ti, Vulkan

### Issue description

The GLTF import system has changed substantially between Godot 3.4 and 4.0. Although the 4.0 system has many advantages, one of the more useful import options from the 3.4 system is currently missing.

In Godot 3.4, there is an option to import each object in a GLTF file as a separate scene, which means that you can store an entire set of related assets in a single file and import/reimport them with very little effort. In Godot 4.0, the option to set the storage type is missing, and no extra files are created when a GLTF is imported, which is quite a different experience in terms of ease-of-use compared to 3.4.

Here is the option I am talking about shown in Godot 3.4:
![godot_importer-sub-scenes](https://user-images.githubusercontent.com/1097744/147622757-a2dc80bf-9164-4c49-907d-1e4ad7033382.png)

This is the result of this setting in the File Tree:
![godot_importer-files](https://user-images.githubusercontent.com/1097744/147622789-051be277-d947-4441-bcf9-3e717365ea0c.png)

The file DefaultShapes.glb is imported using the above settings to automatically create many different sub-scenes. In this case, DefaultShapes contains lots of basic shapes that I can use for blocking out levels, and I'm also running them through a Post Import script to set up collision using convex hulls, boxes, spheres and capsules for each object, but without the option to store objects as individual sub-scenes, none of this works at all.

Godot 4.0 does have the option to store individual objects as Meshes, but this needs to be enabled for each object, and the file path needs to be manually set, which is a lot of extra work. Also, this way you can't add collision data automatically via script because mesh resources do not allow the user to specify collision manually and store it with the mesh, which is one of the reasons that being able to store separate scenes is so valuable.

This seems like a fairly fundamental missing feature between 3.4 and 4.0, so if possible, please add the object storage option back into the basic import settings, and additionally, add an option to set the identity transform on each object during import, as it's convenient to lay out assets in Blender so that they can be viewed and edited together, but the root transform should be reset on import unless we are importing a full scene or level. These issues are related, and there is another request for this feature here: #13980

### Steps to reproduce

To compare the feature between Godot 3.4 and Godot 4.0 do the following (note that I've provided a test file attached to this bug report)

3.4
1. Create a new empty project
2. Drag in the attached file (DefaultShapes.glb)
3. In the import settings tab, change 'Storage' to 'Instanced Sub-scenes' 
4. Hit the reimport button. This will create a single scene for each object in the gltf file.

4.0
1. Create a new empty project
2. Drag in the attached file
3. Note that in neither the import settings, nor the advanced settings is an option to import objects as individual scenes.
Perhaps this feature would make sense as an 'Import as:' preset, but currently it is missing.

### Minimal reproduction project

[DefaultShapes.zip](https://github.com/godotengine/godot/files/7787143/DefaultShapes.zip)