Add Promise.allSettled() (#4385)
Add Promise.allSettled() (#4385)

* Add Promise.allSettled()
This patch adds this method to the Promise.json data with what should be
correct version information for Chrome, Edge, and Firefox. There is no
specification URL because currently `allSettled()` is not yet part of the
spec, and the schema rejects the URL of the proposal.

Sources:
* https://tc39.es/proposal-promise-allSettled/
* Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1539694
* Chrome status: https://www.chromestatus.com/features/5547381053456384
* Edge: https://developer.microsoft.com/en-us/microsoft-edge/platform/catalog/?page=1&q=allSettled

* Updated per review comments.
Set Firefox to `false` and added a `spec_url` entry referencing the
proposal. This will need to be updated when `allSettled()` is added
to the spec proper.

mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data