[REF] app: reorganize startup code

The goal of this refactoring is to delay the instantiation of the
component node until we are ready to mount the component. This is useful
so we know if a component will be attached or mounted (which means that
it needs to have a template, or not)

tests/components/props_validation.test.ts

@@ -702,7 +702,7 @@ describe("props validation", () => {
     const app = new App(Parent, { test: true });
     await app.mount(fixture);
     expect(fixture.innerHTML).toBe("12");
-    expect(app.root!.subscriptions).toEqual([{ keys: ["otherValue"], target: obj }]);
+    expect(app.root!.node!.subscriptions).toEqual([{ keys: ["otherValue"], target: obj }]);
   });
 
   test("props are validated whenever component is updated", async () => {