add module declaration for rehypeTruncate

resources/js/types/rehype-truncate.d.ts

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+// Copyright (c) ppy Pty Ltd <[e-mail]>. Licensed under the GNU Affero General Public License v3.0.
+// See the LICENCE file in the repository root for full licence text.
+
+declare module 'rehype-truncate' {
+  import { Plugin } from 'unified';
+
+  const rehypeTruncate: Plugin;
+  export default rehypeTruncate;
+}