Consider allowing use of `ChannelType.PostMessage` in webR config

Hey, I noticed you've updated to webR 0.2.1. That version contains a new communication channel, named PostMessage, which works even if the web page is not Cross-Origin Isolated, without the (admin & performance) overhead of setting up the Service Worker channel.

You might consider using it as a fallback in quarto-webr when crossOriginIsolated returns false in JS, in place of the automatic Service Worker-based fallback. Or at least provide it as an option to the user via a yaml option.

Positives:

Easier to setup (no need to bundle webr-serviceworker.js and webr-worker.js)
Works in more places (e.g. RStudio preview pane, VSCode built-in browser)

Negatives:

Some R features based on blocking for input (readline(), menu(), browser()) don't work.
Long running R code cannot be interrupted.

If you'd like to experiment, you should be able to make the following (temporary!) change to webr-init.html to import ChannelType and force webR to use ChannelType.PostMessage:

  // Retrieve the webr.mjs
  import { WebR, ChannelType } from "https://webr.r-wasm.org/v0.2.1/webr.mjs";

  // Populate WebR options with defaults or new values based on 
  // webr meta
  globalThis.webR = new WebR({
    "baseURL": "{{BASEURL}}",
    "serviceWorkerUrl": "{{SERVICEWORKERURL}}",
    "homedir": "{{HOMEDIR}}",
    "channelType": ChannelType.PostMessage,
  });

coatless / quarto-webr

Consider allowing use of `ChannelType.PostMessage` in webR config #31