[Bf-usd] Emissive Color export

[Ashley Ruglys]

Hey guys, first time contributor here. I just created this PR to add
support for emissive colors as per the UsdPreviewSurface spec:
https://projects.blender.org/blender/blender/pulls/104490. I'm planning on
adding a few more contributions for things like automatically adding a root
node and fixing some texture imports (e.g.
https://projects.blender.org/blender/blender/issues/100452).
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