Rename from KHR_audio to KHR_audio_emitter and add example file

[aaronfranke]

See omigroup/gltf-extensions#205 and omigroup/gltf-extensions#207

See KhronosGroup#2137 (review)

This PR renames the extension to KHR_audio_emitter to be more similar to the names of the extensions this originally came from (OMI_audio_emitter and MSFT_audio_emitter).

Reasoning:

1. Matches the recommended naming structure: https://github.com/KhronosGroup/glTF/tree/main/extensions#naming

Names SHOULD be structured as <PREFIX>_<scope>_<feature>, where scope is an existing glTF concept (e.g. mesh, texture, image) and feature describes the functionality being added within that scope. This structure is recommended, but not required.

2. Gives us the opportunity to have more extensions under the KHR_audio_ scope. For example, I could see possible KHR_audio_listener, KHR_audio_material, KHR_audio_reverb, etc extensions. And of course, we would want extensions for specific audio types, like KHR_audio_opus, KHR_audio_vorbis, etc. Then KHR_audio would refer to a whole family of extensions, instead of a single extension.

3. Is more descriptive of what the extension does. KHR_audio doesn't say what it actually does to the audio. Does it do everything with audio? No. Does it listen to audio? No. Does it define how audio reverbs in an area or off of objects? No. Does it define footstep sounds? No. KHR_audio_emitter describes that it's an extension for emitting audio.

I don't see a strong reason to use the name KHR_audio.

[antpb]

As per our discussions in past meetings, the rename is agreed by OMI members