asyncdispatch: export Callback (#14042) [backport]
asyncdispatch: export Callback (#14042) [backport]

This let us see the definition of `Callback` in docs, which is required
to even make use of asyncdispatch.

Ref #13539.
dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim