Add allowInterrupt :: IO () (#4810)

Add allowInterrupt :: IO ()  (#4810)

docs:

-- | When invoked inside 'mask', this function allows a blocked
-- asynchronous exception to be raised, if one exists.  It is
-- equivalent to performing an interruptible operation (see
-- #interruptible#), but does not involve any actual blocking.
--
-- When called outside 'mask', or inside 'uninterruptibleMask', this
-- function has no effect.