Back/forward-compatibility for Sourcegraph extensions

[sqs]

From @lguychard (copied from sourcegraph/code-intel-extensions#8 (comment)):

We should plan to handle the issue of extension API breakage differently - IMO there should be no backwards compatibility handling in extension code, it should be handled at the registry/extension host level.

eg.:

If the user has activated the Sourcegraph extension E, which is only compatible with extension API vN, and his browser extension packages extension API vN+1, he receives a notification that "Incompatible extension E could not be activated"
If the user's browser extension packages extension API vN, the extension registry will provide the latest version of E that was compatible with extension API vN, even if the very latest version of E on the registry was coded against vN+1

[lguychard]

Prerequisite: #2080

[felixfbecker]

If the user's browser extension packages extension API vN, the extension registry will provide the latest version of E that was compatible with extension API vN, even if the very latest version of E on the registry was coded against vN+1

I doubt this will work in practice because the old version can be incompatible for many other reasons than just the extension API - GraphQL changes, langserver changes, raw API changes...