Support using AbortController with FileAPI

Hey,

It would be useful to use the web platform standard cancellation method to abort readers.
Other web platform code (like fetch) supports cancellation through signals.

Code before:

async function readFile(file, { signal }) {
  var reader = new FileReader();
  const handler = e => reader.abort();
  signal.addEventListener('abort', handler, { once: true });
  reader.addEventListener('load', () => signal.removeEventListener('abort', handler));
  reader.addEventListener('error', () => signal.removeEventListener('abort', handler));
  const p = new Promise((resolve, reject) => {
    reader.onload = (e) => resolve(e.target.result);
    reader.onerror = (e) => reject(reader.error);
  });
  reader.readAsText(file);
  return await p;
}

Possible suggested change, assuming signal support:

async function readFile(file, { signal }) {
  var reader = new FileReader({ signal });
  const p = new Promise((resolve, reject) => {
    reader.onload = (e) => resolve(e.target.result);
    reader.onerror = (e) => reject(reader.error);
  });
  reader.readAsText(file);
  return await p;
}

Start with the FileReader and AbortController examples in the issue, then read the FileAPI specification entry points for FileReader construction, reading, aborting, and error handling. Define the signal-support behavior and its interaction with existing reader events; done means the specification consistently describes cancellation and its observable results.