Unify AggregateError ctor with Error ctor

[marjakh]

1) The order of setting fields: 1) create object 2) toString(message) & set "message" 3) IterableToList(errors) & set "errors".

• This order is more natural (first "superclass fields", then "subclass fields")
• This enables engines to reuse the code for creating Error objects

2) Unify handling of the message parameter with Error ctor

Having the exact same spec text makes it clear that these ctors do the exact same thing w/ the message parameter.

[mathiasbynens]

This gained consensus at the June 2020 TC39 meeting.

@marjakh Would you mind rebasing so we can get this merged?