Blender 2.91 support and more components!

• Update to support Blender 2.91, namely fixing lightmap export. This version of the exporter will NOT work with older versions of Blender. If you need to keep using 2.83 you should use the old version of the exporter. Versions in between 2.83 and 2.91 may work with one or the other, but are not considered supported.
• Fix nav-mesh component not exporting properly. You should now be able to specify custom nav meshes by adding the nav-mesh component to a mesh. You probably also want to add the visible component to it, to prevent it from showing up (though having it be visible could be useful for debugging).
• Add background, fog, audio-settings, image, video, audio, link, and visible components (thanks @rawnsley for the PRs!). The media elements have issue when being used in non-scene gltfs. We will need to fix this on the Hubs side.
• Some misc code cleanup of hacks no longer needed due to gltf exporter updates.