Bug: GLTF 1.0 Importer emission not loaded

**Describe the bug**
GLTF Importer does not ever load in emission colors/textures

**To Reproduce**
https://github.com/assimp/assimp/blob/master/code/AssetLib/glTF/glTFAsset.inl

It is noticed that emission is not loaded in as a TexProperty with diffuse, ambient, and specular.

I have not had time to confirm, but I looked at all references and saw no other loading for the value. I just want to make note of this.

**Expected behavior**
Emission should be loaded in as per normal

**Additional context**
This was conducted by code read-through. I am not sure it is good to report read-through bugs as they haven't yet caused any issues, but thought I would mention this.

Start by reading code/AssetLib/glTF/glTFAsset.inl and trace how diffuse, ambient, and specular TexProperty values are loaded. Check the existing glTF importer tests or fixtures for material-property coverage, then confirm that a GLTF 1.0 material's emission color and texture are represented after import. Done means the behavior is covered by a regression test.

Written by the indexing model from the issue text.