Add XREnvironmentBlendMode (#4984)
Add XREnvironmentBlendMode (#4984)

* Add XREnvironmentBlendMode
Add XREnvironmentBlendMode.json to cover this
enum type.

Source: https://immersive-web.github.io/webxr-ar-module/#xrenvironmentblendmode-enum
*

* Updates per Joe's feedback

chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data