@types/ckeditor__ckeditor5-heading/src/headingbuttonsui.d.ts

import { Plugin } from "@ckeditor/ckeditor5-core";

export default class HeadingButtonsUI extends Plugin {
    init(): void;
}

declare module '@ckeditor/ckeditor5-core/src/plugincollection' {
    interface Plugins {
        HeadingButtonsUI: HeadingButtonsUI;
    }
}