Partially resolves: https://github.com/zed-industries/zed/issues/7785

Right now, we can engage `Only show installed`, but I've been wanting to
be able to filter down to just uninstalled extensions too, so I can
browse things I don't have. I changed this to have 2 checkboxes,
`Installed` and `Not installed` and both are on by default. You deselect
them to filter down.

<img width="1608" alt="SCR-20240225-etyg"
src="https://github.com/zed-industries/zed/assets/19867440/e2267651-ff86-437b-ba59-89f3d338ea02">

Release Notes:

- Allow extensions list to be filtered down to both installed and not
installed.