draft execution environment for WebGPU, including WebGPU env spec for SPIR-V

Hi,

In a recent meeting Google promised to produce a draft WebGPU environment
spec for SPIR-V.

Today we created a new GitHub repo for all documents related to SPIR-V use
in WebGPU.  It has a first draft of the promised environment spec.  Please
see
https://github.com/gpuweb/spirv-execution-env/blob/master/execution-env.md

In writing the draft we realized there were considerations on how WebGPU
would execute shaders, independent of shader language. So the document is
split in to several parts: A general part that always would apply, and a
part that is a WebGPU environment spec for SPIR-V.

Many items are marked "TBD", and most of them will be resolved once the
community group determines exactly what features are supported by WebGPU.

This is ready for initial review and discussion by the community group.

thanks,
david