feat: prepare for TS defs generation

[web-padawan]

Fixes #220

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

import {IronResizableBehavior} from '@polymer/iron-resizable-behavior/iron-resizable-behavior.js';

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

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

import {FlattenedNodesObserver} from '@polymer/polymer/lib/utils/flattened-nodes-observer.js';

import {mixinBehaviors} from '@polymer/polymer/lib/legacy/class.js';

declare class SelectElement extends
  ElementMixin(
  ControlStateMixin(
  ThemableMixin(
  PolymerElement))) {
  readonly focusElement: HTMLElement;
  opened: boolean;
  renderer: SelectRenderer|undefined;
  errorMessage: string;
  label: string|null|undefined;
  value: string;
  required: boolean|null|undefined;
  invalid: boolean;
  name: string|null|undefined;
  placeholder: string|null|undefined;
  readonly: boolean;
  ready(): void;
  connectedCallback(): void;
  disconnectedCallback(): void;
  _setFocused(focused: boolean): void;
  render(): void;
  notifyResize(): void;
  _onKeyDown(e: KeyboardEvent): void;
  _onKeyDownInside(e: KeyboardEvent): void;
  validate(): boolean;
}

declare global {
  interface HTMLElementTagNameMap {
    "vaadin-select": SelectElement;
  }
}

export {SelectElement};

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