Proposal for adding shader function name to PSV0 part (#112)

This proposes we add a new EntryFunctionName field for PSV0 to encode the entry name for D3D12 runtime use.

There is a lot of PSV0 documented here now, which could serve as a starting point for a new PSV0 spec.
If that's done, the sections documenting existing PSV0 structure could be moved out of this proposal.

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Co-authored-by: Chris B <[email protected]>