Add .allSettled() method

[sindresorhus]

Like https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled

[Richienb]

Before adding this, is it possible to simplify the types so that .all doesn't require 10 separate definitions?

[sindresorhus]

That would be nice, but I'm not sure if it's possible. TypeScript did receive variadic tuple support, but not sure if it solves all the same problems or whether it's as type safe.

• Proposal: Variadic Kinds -- Give specific types to variadic functions microsoft/TypeScript#5453
• https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#variadic-tuple-types

[privatenumber]

This one is a little more complicated but I think it should be possible. Precedent in p-all: sindresorhus/p-all#9

[Richienb]

Reworking those typings for Iterables like Sets might be a challenge because of how P in keyof Task may yield get, set, has, etc. instead of the indexes we want.