Add support for updating config on-the-fly with postMessage

[laymonage]

Fix #134.

This PR adds support for updating giscus config on-the-fly with iframe.contentWindow.postMessage.

Example usage:

interface ISetConfigMessage {
  setConfig: {
    theme?: string;
    repo?: string;
    term?: string;
    number?: number;
    category?: string;
    reactionsEnabled?: boolean;
    emitMetadata?: boolean;
  };
}

function sendMessage<T>(message: T) {
  const iframe = document.querySelector<HTMLIFrameElement>('iframe.giscus-frame');
  if (!iframe) return;
  iframe.contentWindow.postMessage({ giscus: message }, location.origin);
}

sendMessage({
  setConfig: {
    theme: 'https://giscus.app/themes/custom_example.css',
    reactionsEnabled: false,
  }
} as ISetConfigMessage);

This can be used to dynamically change giscus theme based on the parent window's theme (utterance/utterances#427, utterance/utterances#549). It also lets you change the discussion without having to reload the <iframe> or <script> (utterance/utterances#359).

[vercel]

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🔍 Inspect: https://vercel.com/laymonage/giscus/6bDKaVjJzsFsZYKQrhU1yhxTV94B
✅ Preview: https://giscus-git-setconfig-message-laymonage.vercel.app