♻️(react) make className standard across components

The className prop was sometimes set onto the nested element and
sometimes on the container element, which was not consistent. Now
we always set the className onto the upmost element.

packages/react/src/components/Alert/index.spec.tsx

@@ -249,4 +249,11 @@ describe("<Alert/>", () => {
       screen.queryByText("Additional information"),
     ).not.toBeInTheDocument();
   });
+
+  it("renders with className", async () => {
+    render(<Alert className="my-custom-class" />);
+    expect(
+      document.querySelector(".c__alert.my-custom-class"),
+    ).toBeInTheDocument();
+  });
 });