feat: prepare for TS defs generation [skip ci]

[web-padawan]

Fixes #145

Generated file vaadin-button.d.ts looks like this:

import {PolymerElement} from '@polymer/polymer/polymer-element.js';

import {GestureEventListeners} from '@polymer/polymer/lib/mixins/gesture-event-listeners.js';

import {ThemableMixin} from '@vaadin/vaadin-themable-mixin/vaadin-themable-mixin.js';

import {ControlStateMixin} from '@vaadin/vaadin-control-state-mixin/vaadin-control-state-mixin.js';

import {ElementMixin} from '@vaadin/vaadin-element-mixin/vaadin-element-mixin.js';

import {html} from '@polymer/polymer/lib/utils/html-tag.js';

import {addListener} from '@polymer/polymer/lib/utils/gestures.js';

declare class ButtonElement extends
  ElementMixin(
  ControlStateMixin(
  ThemableMixin(
  GestureEventListeners(
  PolymerElement)))) {
  readonly focusElement: Element|null;
  ready(): void;
  disconnectedCallback(): void;
}

declare global {
  interface HTMLElementTagNameMap {
    "vaadin-button": ButtonElement;
  }
}

export {ButtonElement};