repl: Factor out ReplStore (#14970)

This PR factors a `ReplStore` out of the `RuntimePanel`.

Since we're planning to remove the `RuntimePanel` and replace it with an
ephemeral tab that can be opened, we need the kernel specifications and
sessions to have somewhere long-lived that they can reside in.

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -167,7 +167,7 @@ fn init_common(app_state: Arc<AppState>, cx: &mut AppContext) {
     supermaven::init(app_state.client.clone(), cx);
     inline_completion_registry::init(app_state.client.telemetry().clone(), cx);
     assistant::init(app_state.fs.clone(), app_state.client.clone(), cx);
-    repl::init(cx);
+    repl::init(app_state.fs.clone(), cx);
     extension::init(
         app_state.fs.clone(),
         app_state.client.clone(),