Add checks to the validator for WebGPU specific details.

Draft of the spec can be found at https://github.com/gpuweb/spirv-execution-env/blob/master/execution-env.md

Due to the draft nature of the spec, not all of the rules in it are defined enough yet to be actionable, so more issues will be created as new rules are completed. Additionally there may be changes to the spec that require updating already written validation code, those should be handled by creating new issues and patching, instead of trying to revert and re-land.