feat: prepare for TS defs generation

[Haprog]

Fixes #35

Generated TS definitions look like this:

@types/interfaces.d.ts

import { DatePickerI18n } from '@vaadin/vaadin-date-picker/@types/interfaces';
import { TimePickerI18n } from '@vaadin/vaadin-time-picker/@types/interfaces';

export interface DateTimePickerI18n extends DatePickerI18n, TimePickerI18n {}

vaadin-date-time-picker.d.ts

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

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

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

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

declare class DateTimePicker extends
  ElementMixin(
  ThemableMixin(
  PolymerElement)) {

  name: string|null|undefined;
  invalid: boolean;
  required: boolean;
  errorMessage: string|null|undefined;
  value: string;
  min: string|undefined;
  max: string|undefined;
  datePlaceholder: string|null|undefined;
  timePlaceholder: string|null|undefined;
  step: number|null|undefined;
  initialPosition: string|null|undefined;
  showWeekNumbers: boolean|null|undefined;
  label: string;
  autoOpenDisabled: boolean|null|undefined;
  disabled: boolean;
  readonly: boolean;
  autofocus: boolean;
  i18n: DateTimePickerI18n;

  ready(): void;
  focus(): void;
  validate(): boolean;
  checkValidity(): boolean;
}

declare global {
  interface HTMLElementTagNameMap {
    "vaadin-date-time-picker": DateTimePicker;
  }
}

export {DateTimePicker};

import {DateTimePickerI18n} from '../@types/interfaces';

[Haprog]

This is not needed for TS defs, but I added it as a convenience so that the IDE can help with this type even when working with the Polymer 2 sources since TimePickerTime is used internally.

[Haprog]

I checked that the visual test failures already affect master so not broken by this PR. Problem is RTL style updates in date-picker and some issue with date picker border radius only affecting Lumo version. I'll make a separate PR for fixing those.