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/zed.rs

@@ -3417,7 +3417,7 @@ mod tests {
             outline_panel::init((), cx);
             terminal_view::init(cx);
             assistant::init(app_state.fs.clone(), app_state.client.clone(), cx);
-            repl::init(cx);
+            repl::init(app_state.fs.clone(), cx);
             tasks_ui::init(cx);
             initialize_workspace(app_state.clone(), cx);
             app_state