Add types for stage 3 "set methods" proposal

[bakkot]

⚙ Compilation target

esnext

⚙ Library

esnext

Missing / Incorrect Definition

Set.prototype.intersection(other)
Set.prototype.union(other)
Set.prototype.difference(other)
Set.prototype.symmetricDifference(other)
Set.prototype.isSubsetOf(other)
Set.prototype.isSupersetOf(other)
Set.prototype.isDisjointFrom(other)

Sample Code

setInstance.intersection(other)
setInstance.union(other)
setInstance.difference(other)
setInstance.symmetricDifference(other)
setInstance.isSubsetOf(other)
setInstance.isSupersetOf(other)
setInstance.isDisjointFrom(other)

Documentation Link

https://github.com/tc39/proposal-set-methods

Currently stage 3, but it's shipping in Safari and will hit stable Chrome in a couple weeks, so I'll be asking for stage 4 at the April meeting.

I'll go ahead and submit a PR.