Closes #12163
Supersedes https://github.com/zed-industries/zed/pull/56941

This PR changes the way we show installed extensions when offline -
instead of basing it on the remote extensions list, we now compute a
"displayed_extensions" list based on the local state and use that as the
source of truth.

This enables us to fix the issues listed below, as we are no longer
depending on the remote info, but instead use what we have at our hands
as the source of truth in cases where the server response is either not
available or incomplete (we truncate currently and will add pagination
at some point, so this is nice to have in either case).

Release Notes:

- Installed extensions will now also show in the extensions tab when
offline.
- Fixed a bug where some extensions would not show in the "Installed"
tab despite being installed.