chore: Extract assets module out of zed crate. (#3957)

This essentially shaves off about 10% off of an incremental build after
project change and potentially more if you're changing stuff like
`welcome` that's very close to the `zed` crate in the dep graph. That's
because macro expansion takes place even in incremental builds it seems?
And zed (lib) + zed (bin) could take up to 4 seconds out of an
incremental build, which is a *lot* in a 10s build. In reality though it
shaves 1 second off of 5 seconds incremental 'welcome'/ 1s off of 10s
'project' builds.

Note that we had `assets` crate in the past (removed in #2575 /cc
@maxbrunsfeld), but this is a bit different, because `assets` is a
dependency of *just* zed and nothing else. We essentially cache macro
expansion results ourselves.

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -1,11 +1,9 @@
 mod app_menus;
-mod assets;
 pub mod languages;
 mod only_instance;
 mod open_listener;
 
 pub use app_menus::*;
-pub use assets::*;
 use assistant::AssistantPanel;
 use breadcrumbs::Breadcrumbs;
 use collections::VecDeque;
@@ -18,6 +16,7 @@ pub use only_instance::*;
 pub use open_listener::*;
 
 use anyhow::{anyhow, Context as _};
+use assets::Assets;
 use futures::{channel::mpsc, select_biased, StreamExt};
 use project_panel::ProjectPanel;
 use quick_action_bar::QuickActionBar;