feat: prepare for TS defs generation

[pekam]

Closes #141

Generated TS definitions look like this:

vaadin-time-picker.d.ts

(updated)

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 {IronA11yKeysBehavior} from '@polymer/iron-a11y-keys-behavior/iron-a11y-keys-behavior.js';
import {html} from '@polymer/polymer/lib/utils/html-tag.js';
import {ControlStateMixin} from '@vaadin/vaadin-control-state-mixin/vaadin-control-state-mixin.js';

declare class TimePickerElement extends
  ElementMixin(
  ControlStateMixin(
  ThemableMixin(
  PolymerElement))) {

  readonly focusElement: HTMLElement;
  disabled: boolean;
  name: string|null|undefined;
  value: string;
  label: string|null|undefined;
  required: boolean;
  preventInvalidInput: boolean|null|undefined;
  pattern: string|null|undefined;
  errorMessage: string|null|undefined;
  placeholder: string;
  readonly: boolean;
  invalid: boolean;
  min: string;
  max: string;
  step: number|null|undefined;
  clearButtonVisible: boolean;
  autoOpenDisabled: boolean|null|undefined;
  i18n: TimePickerI18n;
  ready(): void;
  validate(): boolean;
  _timeAllowed(time: TimePickerTime): boolean;
  checkValidity(): boolean;
}

declare global {

  interface HTMLElementTagNameMap {
    "vaadin-time-picker": TimePickerElement;
  }
}

export {TimePickerElement};
import {TimePickerI18n} from '../@types/interfaces';
import {TimePickerTime} from '../@types/interfaces';

[pekam]

I wasn't sure what to do with this. I feel like this _getInputElement should be private and the __inputElement getter below should be protected, but their number of underscores implies the exact opposite. So now they're both private.

[web-padawan]

Let's add @type {string} for placeholder, min and max properties.

[Haprog]

1. Properties value, placeholder, min, max should have @type {string}
2. Double underscored properties/methods should have explicit @private: __dropdownItems, __validDayDivisor(), __onKeyDown(), __onArrowPressWithStep(), __dispatchStep(), __getMsec(), __getSec(), __addStep(), __updateDropdownItems(), __generateDropdownList(), __adjustValue(), __valueChanged(), __onInputChange(), __updateValue(), __updateInputValue(), __validateTime(), __getStepSegment(), __formatISO(), __parseISO(), __inputElement(), __dropdownElement()
3. Single underscored properties/methods should have explicit @protected: ready()