Merge SPIR-V environment specification into API specification

[kpet]

The specification tooling now has some support for writing unified specification that comprise both core features and extensions. A next logical step seems to be to merge the SPIR-V environment specification into the API specification, likely as a new chapter or normative appendix. As an example the Vulkan specification fully describes the SPIR-V environment rules in the same document as the API. This makes it much easier to cross-reference the two and make the documents generally easier to use for implementers and application writes.