fix: Import ha-expansion-panel in ha-form-expandable

The current usages of ha-form-expandable only work correctly because
ha-expansion-panel is already imported somewhere else by coincidence.

This adds an explicit import to avoid breakages when using
ha-form-expandable in a standalone context (e.g. within ./hassio).

Author: cr7pt0gr4ph7

src/components/ha-form/ha-form-expandable.ts

@@ -2,6 +2,7 @@ import { css, html, LitElement, nothing } from "lit";
 import { customElement, property } from "lit/decorators";
 import type { HomeAssistant } from "../../types";
 import "./ha-form";
+import "../ha-expansion-panel";
 import type {
   HaFormDataContainer,
   HaFormElement,