Add 'webvr' to "Permission Registry" enum

[cvan]

https://w3c.github.io/permissions/#permission-registry

(See immersive-web/webxr#30 (comment) for context.)

[jyasskin]

Please add a <dfn enum-value for="PermissionName">"vr"</dfn> to the WebVR spec (or whatever name you think is reasonable), with entries for the algorithms and types from the Permissions Registry section. Once that's committed, we can add a value to the enum here that'll link to it.

You can imitate WebBluetoothCG/web-bluetooth@5a4c17a#diff-ec9cfa5f3f35ec1f84feb2e59686c34dR1209 (Eventually https://webbluetoothcg.github.io/web-bluetooth/#dom-permissionname-bluetooth, but that hasn't been updated yet for some recent changes to the Permissions spec.)