Add kernel detection for language support of runnable markdown cells (#29664)

Closes #27757

Release Notes:

- List of runnable markdown cells is now based on detected jupyter
kernels instead of hardcoded to Python and TypeScript

crates/repl/src/repl_store.rs

@@ -279,4 +279,14 @@ impl ReplStore {
     pub fn remove_session(&mut self, entity_id: EntityId) {
         self.sessions.remove(&entity_id);
     }
+
+    #[cfg(test)]
+    pub fn set_kernel_specs_for_testing(
+        &mut self,
+        specs: Vec<KernelSpecification>,
+        cx: &mut Context<Self>,
+    ) {
+        self.kernel_specifications = specs;
+        cx.notify();
+    }
 }