Add interface for Lang['props'] and quasarLanguagePack

[Dtsiantaris]

While developing an app-extension, having custom components with labels,text etc. required to extend the Quasar Language Pack. This is fairly easy to do as you can do this:

import { Lang } from 'quasar'

Lang.props.customProp = 'Custom Value'

And then you can access these custom values in your template with $q.lang.customProp.

Typescript throws errors because of this so I tried to augment these interfaces:

import 'quasar'
declare module 'quasar' {
  interface Lang {
    props: {
      customProp: string
    }
  }

  interface GlobalQuasarLanguage {
    customProp: string
  }
}

The GlobalQuasarLanguage gets augmented correctly. But the props inside Lang does not.
Taking a look at quasar's index.d.ts I found that the produced interface for Lang has the props field typed inline(And typescript does not play well with augmenting inline typed fields).

export interface Lang {
  /**
   * Quasar language pack
   */
  props: {
    /**
     * The ISO name of the Quasar language pack
     */
    isoName: string;
    /**
     * The native name of the Quasar language pack
     */
    nativeName: string;
    ....///

If an extra interface called LangProps for example would exist, I could augment that instead. Using this interface would remove duplication of typing, because inside set() of the Lang interface, the quasarLanguagePack argument is of the same exact type as props.

export interface Lang {
  /**
   * Quasar language pack
   */
  props: {/*Same type as quasarLanguagePack, excluding `rtl` which is optional on the second case*/};
  /**
   * Set another Quasar Language Pack
   * @param quasarLanguagePack Usually you will import such an object directly from quasar (eg: import qIconSet from 'quasar/lang/<lang-name>')
   * @param ssrContent Required for SSR only
   */
  set: (
    quasarLanguagePack: {
      /**
       * The ISO name of the Quasar language pack
       */
      isoName: string;
      /**
       * The native name of the Quasar language pack
       */
      nativeName: string;
      ...///

As Lang.props is something that CAN be extended, I think it would be more powerful if also typed more conveniently.