feat: prepare for TS defs generation

[Haprog]

Fixes #127

Generated TS definitions look like this:

vaadin-board.d.ts

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

declare class BoardElement extends
  ElementMixin(
  PolymerElement) {

  static _finalizeClass(): void;
  redraw(): void;
}

declare global {
  interface HTMLElementTagNameMap {
    "vaadin-board": BoardElement;
  }
}

export {BoardElement};

vaadin-board-row.d.ts

import {PolymerElement} from '@polymer/polymer/polymer-element.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 {afterNextRender} from '@polymer/polymer/lib/utils/render-status.js';

import {DomRepeat} from '@polymer/polymer/lib/elements/dom-repeat.js';

import {DomIf} from '@polymer/polymer/lib/elements/dom-if.js';

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

declare class BoardRowElement extends
  ElementMixin(
  PolymerElement) {

  ready(): void;
  connectedCallback(): void;
  redraw(): void;
}

declare global {
  interface HTMLElementTagNameMap {
    "vaadin-board-row": BoardRowElement;
  }
}

export {BoardRowElement};

[CLAassistant]

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