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/main.rs

@@ -16,6 +16,7 @@ use isahc::{prelude::Configurable, Request};
 use language::LanguageRegistry;
 use log::LevelFilter;
 
+use assets::Assets;
 use node_runtime::RealNodeRuntime;
 use parking_lot::Mutex;
 use serde::{Deserialize, Serialize};
@@ -49,8 +50,8 @@ use welcome::{show_welcome_view, BaseKeymap, FIRST_OPEN};
 use workspace::{AppState, WorkspaceStore};
 use zed::{
     app_menus, build_window_options, ensure_only_instance, handle_cli_connection,
-    handle_keymap_file_changes, initialize_workspace, languages, Assets, IsOnlyInstance,
-    OpenListener, OpenRequest,
+    handle_keymap_file_changes, initialize_workspace, languages, IsOnlyInstance, OpenListener,
+    OpenRequest,
 };
 
 fn main() {