allow "Throw ..." as sugar for "Return ThrowCompletion(...)"

[michaelficarra]

Personally this notation would seem really intuitive. It looks like we already have 5 cases in the spec that could use it and #3655 will be introducing another 4. This comes up any time an error object is created in a separate step from where it's thrown, which happens whenever you need to do cleanup or augment the error, or whenever the error is provided by another AO.

[ljharb]

Seems like that's basically https://tc39.es/ecma262/#sec-throw-an-exception, but adapted to be slightly more generic?

[michaelficarra]

Yes, a more generic sugar than that one.