feat: prepare for TS defs generation [skip ci]

[web-padawan]

Fixes #63

Generated TS definitions look like this:

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

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

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

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

declare class CookieConsentElement extends
  ElementMixin(
  ThemableMixin(
  PolymerElement)) {
  message: string;
  dismiss: string;
  learnMore: string;
  learnMoreLink: string;
  position: CookieConsentPosition;
  cookieName: string;
  static _finalizeClass(): void;
  connectedCallback(): void;
  disconnectedCallback(): void;
  _show(): void;
}

declare global {
  interface HTMLElementTagNameMap {
    "vaadin-cookie-consent": CookieConsentElement;
  }
}

export {CookieConsentElement};

import {CookieConsentPosition} from '../@types/interfaces';