Combine all themes into a single playground

[epicfaace]

Fixes #1536.

Description

• Make a separate package called @rjsf/playground. This exports a Playground component that takes an object of themes in its props.
• This package will be published to npm. It will have a single "playground" within it which will have all themes, such as the following:

import React from "react";
import { Theme as MuiTheme } from "rjsf-material-ui";
import Playground from "../src/index";
import { render } from "react-dom";

const themes = {
  default: {
    stylesheet:
      "//maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css",
    theme: {},
  },
  "material-ui": {
    stylesheet: "",
    theme: MuiTheme,
  },
};

render(<Playground themes={themes} />, document.getElementById("app"));

• Each package can have a "development" playground with the following code:

import React from "react";
import { render } from "react-dom";
import Playground from "@rjsf-team/playground";

const themes = [
  {
    default: {
      stylesheet:
        "//maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css",
      theme: {},
    },
  },
];

render(<Playground themes={themes} />, document.getElementById("app"));

[epicfaace]

How it looks like so far:

[edi9999]

Looks good to me