use Fetch API instead of XHR

[uneco]

from:

export const httpGet = async(
  url: string,
  header: { [s: string]: string } = {}
): Promise<XMLHttpRequest> => {
  return new Promise<XMLHttpRequest>((resolve, _reject) => {
    const xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function (event) {
      if (xhr.readyState !== 4) return;
      return resolve(xhr); // OK
    };
    xhr.open('GET', url, true); // Async
    Object.keys(header).forEach(function (key) {
      xhr.setRequestHeader( key, header[key] );
    });
    xhr.withCredentials = true;
    xhr.send();
  });
};

to:

export const httpGet = async (url: string, headers: Headers = {}) => {
  return fetch(url, {
    headers,
    credentials: 'include',
  })
}

[takanakahiko]

@uneco I was thinking of making the same topic Issue too.
PRs welcome!