This function allows you to load javascript from a remote url with an extension content script, and have the loaded javascript execute within the content script's isolated world.

loadInContentScript.js

async function loadInContentScript(url) {
  // Note that this extension must either have permission to the domain of this
  // URL, or the URL must be served with the correct CORS HTTP headers to allow
  // this script to fetch it (ie. "Access-Control-Allow-Origin: *").
  const response = await fetch(url);
  const scriptText = await response.text();

  // Let Chrome devtools know where this code came from.
  const codeToRun = scriptText + "\n//# sourceURL=" + url + "\n";

  // Put eval into a variable first and then call it so we get "indirect eval"
  // behavior. A direct eval call is actually syntactically-special and has
  // effects that we don't want. Code passed to a direct eval call has access
  // to all of the local variables of the call site, and this prevents the
  // javascript engine from many optimizations.
  const indirectEval = eval;
  indirectEval(codeToRun);
}