[charter] add WebXR-WebGPU-Binding to normative deliverables

[himorin]

memo for 2026 rechartering
we need to formally add WebXR-WebGPU-Binding spec to normative deliverables. which is of course in scope, but it seems we haven't added to the list since we were unsure whether this module will be inserted into other specs (Core or Layers) or not.