fixes #3151

Motivation: to build docker image with firestore emulator and ui we can prefetch firestore emulator with help of setup:emulators:firestore but there is no such command for ui, so it is being downloaded each time container starts.

Workaround: meanwhile workaround is to

ADD https://storage.googleapis.com/firebase-preview-drop/emulator/ui-v1.4.1.zip /root/.cache/firebase/emulators/ui-v1.4.1.zip
RUN unzip /root/.cache/firebase/emulators/ui-v1.4.1.zip -d /root/.cache/firebase/emulators/ui-v1.4.1

What was done:

If I understood everything correctly all we need is add

src/commands/setup-emulators-ui.js

"use strict";

const { Command } = require("../command");
const { Emulators } = require("../emulator/types");
const { downloadEmulator } = require("../emulator/download");

const NAME = Emulators.UI;

module.exports = new Command(`setup:emulators:${NAME}`)
  .description(`downloads the ${NAME} emulator`)
  .action((options) => {
    return downloadEmulator(NAME);
  });

And add it to src/commands/index.js