KHR_materials_emissive_strength added.

* initial commit for materials_emissive
* change dir name
* Update physical units, set default to 100.
* Rename folder
* Add emissive strength schema.
* Fix leftover value from earlier draft.
* Contributor list update
* Revise default value and units descriptions based on recent discussion.
* Update description to match jon-adoc branch.
* Remove text redundant with core spec.
* Remove section number, per offline review
* Update to Release Candidate status
* Update copyright message from main glTF registry.
* Update contributors
* fix: Added myself as contributor to KHR_materials_emissive_strength (#2138)
* Changes per review
* Mark KHR_materials_emissive_strength as ratified.

Co-authored-by: Ben Houston <[email protected]>
Co-authored-by: Andreas Atteneder <[email protected]>