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/zed/src/main.rs

@@ -402,7 +402,7 @@ fn main() {
             app_state.client.telemetry().clone(),
             cx,
         );
-        extension::init(
+        extension_host::init(
             app_state.fs.clone(),
             app_state.client.clone(),
             app_state.node_runtime.clone(),