Add a package with JSON response middleware

[gigabo]

This just handles some of the boilerplate involved with setting up a JSON
endpoint in React Server.

Example (from the README):

import JsonResponseMiddleware from "react-server-middleware-json-response"

export default class DataEndpoint {
    static middleware() {
        return [JsonResponseMiddleware];
    }

    getResponseData() {
        return { result: "OK!" };
    }
}