Add support for optional channels in Select #12

   def with_optional(self, **optional_receivers: Optional[AsyncIterator[Any]]) -> Select:
       # TODO: check name clashing with self._receivers
       for name, recv in optional_receivers.items():
           self._result[name] = None
           if recv is None:
               continue
           self._receivers[name] = recv
           # can replace __anext__() to anext() (Only Python 3.10>=)
           msg = recv.__anext__()  # pylint: disable=unnecessary-dunder-call
           self._pending.add(asyncio.create_task(msg, name=name))  # type: ignore

       return self

# chan1 can't be None, chan_none can
select = Select(chan1=chan1).with_optional(chan_none=None)
while await select.ready():
    if select.chan1:
        pass
    elif select.chan_none:  # will always be None if chan_none is None
        pass

chan_none = None
optional_channels = {}
if chan_none is not None:
    optional_channels["chan_none"] = chan_none
select = Select(chan1=chan1, **optional_channels)
while await select.ready():
    if select.chan1:  # OK
        pass
    elif chan_none and select.chan_none:  # OK too
        pass

chan_none = Broadcast[int]("chan_none").get_receiver()
select = Select(chan1=chan1, chan_none=chan_none)
while await select.ready():
    if select.chan1:  # OK
        pass
    elif select.chan_none:  # OK too, will never be True
        pass