fix: descriptor extensions handling post-editions

[mkruskal-google]

This extension had virtually no test coverage and was already fairly buggy in proto2/proto3. The addition of editions support broke it completely though, notably #2068 which added some caching that doesn't work in the case of root-less types.

This PR adds some basic test coverage, and cleans up the code to work more smoothly under editions. It is still not safe to use it in the following cases, where it will produce bad results:

• Converting descriptors from one syntax/edition to another
• Loading partial editions descriptors (e.g. calling fromDescriptor on anything but a Root node)

Fixes #2072