Fixes to ESCN mesh import

[fire]

Fixes: #63862

Convert mesh instance 3d to a mesh importer instance 3d. Multi mesh is not converted.

Restore ESCN to a working state even if the Blender importer is slower to be updated.

Restores the tscn to escn usecase.

godotengine/godot-docs#6603

Test Plan

1. take the fox from Renaming a TSCN to ESCN doesn't regenerate lods #63862 or anywhere else
2. clear inheritance
3. save as different tscn
4. rename that tscn to escn
5. check that the lod works by setting it to lod property to 0 and compare.

[fire]

Blend Shape Test Plan

1. take the asset from https://github.com/KhronosGroup/glTF-Sample-Models/blob/master/2.0/AnimatedMorphCube/glTF-Binary/AnimatedMorphCube.glb or anywhere else
2. clear inheritance
3. save as different tscn
4. rename that tscn to escn
5. check that the lod works by setting it to lod property to 0 and compare.

[akien-mga]

Thanks!