Refactor <Header> into TypeScript (#33956)

app/javascript/mastodon/models/dropdown_menu.ts

@@ -0,0 +1,24 @@
+interface BaseMenuItem {
+  text: string;
+  dangerous?: boolean;
+}
+
+interface ActionMenuItem extends BaseMenuItem {
+  action: () => void;
+}
+
+interface LinkMenuItem extends BaseMenuItem {
+  to: string;
+}
+
+interface ExternalLinkMenuItem extends BaseMenuItem {
+  href: string;
+}
+
+export type MenuItem =
+  | ActionMenuItem
+  | LinkMenuItem
+  | ExternalLinkMenuItem
+  | null;
+
+export type DropdownMenu = MenuItem[];