fix: aggregate error v4

[hongaar]

This change makes utils.extractErrors compatible with aggregate-error v4, which changes its implementation to store the errors in an .errors property instead of making the Error instance iterable.

See sindresorhus/aggregate-error@02342f9

Note: could not run the linter due to Error: Cannot find module 'prettier'

[travi]

our work toward converting the core project to esm in #2607 enables us to upgrade to aggregate-error v4. we are very close to being ready to promote that work to latest. do you see this change as being necessary after that change?

[hongaar]

The change itself would still be needed but the test should probably be changed:

// This will now test for the `isFunction(err.errors[Symbol.iterator])` path
t.deepEqual(extractErrors(new AggregateError(errors)), errors);

// Add something like
// Test aggregate-error < v4 which stores errors by making object iterable
const legacyAggregateError = new Error();
legacyAggregateError[Symbol.iterator] = function* () { 
  for (const error of errors) {
    yield error;
  }
}
t.deepEqual(extractErrors(legacyAggregateError), errors);

Happy to contribute a PR once ESM support is merged.

[hongaar]

Closing since latest supports aggregate-error v4. Not sure if there is value in adding backwards compatibility with earlier aggregate-error versions? May be useful when using plugins which depend on earlier versions of aggregate-error. While they won't currently break, they will not be leveraging the aggregate-error functionality. If wanted, I will reopen another PR and add the test from #2631 (comment).