Rename extension crate to extension_host (#20081)

This PR renames the `extension` crate to `extension_host`.

This is to free up the name so that we can create a smaller-scoped
`extension` crate.

Release Notes:

- N/A

crates/activity_indicator/src/activity_indicator.rs

@@ -1,6 +1,6 @@
 use auto_update::{AutoUpdateStatus, AutoUpdater, DismissErrorMessage};
 use editor::Editor;
-use extension::ExtensionStore;
+use extension_host::ExtensionStore;
 use futures::StreamExt;
 use gpui::{
     actions, percentage, Animation, AnimationExt as _, AppContext, CursorStyle, EventEmitter,