Add AggregateError supprt

[shortercode]

Relates to #294. Implements support for the relatively new but widely supported AggregateError type. This is a subclass of Error which supports passing an array of child errors. As an example this is useful if you need to make a series of network requests to load a page, you can return a single error which captures all the related failures.

I figured this would be more controversial than the #296 change so separated the 2. This duplicates some of the changes but only to provide complete implementation for this MR. I thought it best to introduce it as a separate transformer for backwards compatibility.

Adds type check for AggregateError

[Skn0tt]

Hi! I'm generally supportive of this addition. Could you add some tests for it?