Patch Subset Spec Updates

Just PR'd a substantial update to the patch subset spec:
https://github.com/w3c/PFE/pull/4

This update provides text for all of the client and server sections of the
patch subset method. There's a few notable changes from the
earlier protocol design (
https://github.com/w3c/PFE-analysis/blob/main/design/patch_subset_protocol_v3.md
):
- I've dropped the response_type field. Instead the client processing of
the response works by inspecting which fields are set/unset in the response.
- Previously REBASE and PATCH were treated as two separate cases in client
side handling. I've unified these. REBASE operations are now just treated
as a PATCH against an empty file. This also simplifies the patch formats
since we don't need different formats for the patch and rebase operations.
- I've removed most of the detailed descriptions of server side handling
and instead left a more open ended requirement that the server response
must result in the client extending their font to at least cover the
requested codepoints. This leaves lots of flexibility for
server implementations to do things like font version upgrades, prediction,
etc. without needing to explicitly include it in the spec.

With this update the only remaining unwritten sections under patch subset
are the checksum and codepoint reordering sections.