web: Add document highlight providers to API and extension host
Created by: efritz
Part of https://github.com/sourcegraph/sourcegraph/issues/10868. Sibling PRs: https://github.com/sourcegraph/code-intel-extensions/pull/409 and https://github.com/sourcegraph/codeintellify/pull/278
This PR adds the DocumentHighlightProvider to the extension API. The behavior is very straightforward, but there are lot of places that needed to be touched to thread things through to the leaves of the app appropriately. Please look with a close eye!