fix(ISubscription): update type definition of ISubscription::closed

[kwonoj]

Description:

Originated from #2234, doesn't add any code changes but only update type definition for interface clarity.

Currently it's not so benefitable even for TS user since all of codebase just return Subscription everywhere instead of using interfaces, that need to be dealt with separate refactoring.

Related issue (if exists):
#2234

[coveralls]

Coverage remained the same at 97.696% when pulling abd3a2a on kwonoj:chore-isubscription into 9c9870e on ReactiveX:master.

[david-driscoll]

👍 this will encourage TypeScript users to not try to modify this value (as it shouldn't be mutable)

[kwonoj]

@david-driscoll check #2234 as well, this PR is subset of it only updated type definition, while it does updates actual code behaviors.

[benlesh]

since all of codebase just return Subscription everywhere instead of using interfaces, that need to be dealt with separate refactoring.

Technically, a LOT of the codebase returns subclasses of Subscriber. So people could call next on them if they chose too.. however, that's not supported nor is it documented.

I guess what I'm saying is: Hold off on that refactor, there's likely one coming anyhow... especially with the planned scheduling changes.

[jayphelps]

LGTM

[kwonoj]

LGTM

[lock]

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