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/extensions_ui/src/extension_suggest.rs

@@ -4,7 +4,7 @@ use std::sync::{Arc, OnceLock};
 
 use db::kvp::KEY_VALUE_STORE;
 use editor::Editor;
-use extension::ExtensionStore;
+use extension_host::ExtensionStore;
 use gpui::{Model, VisualContext};
 use language::Buffer;
 use ui::{SharedString, ViewContext};