Extend `selectors` module to support platform-specific events

The selectors module API is deliberately very narrow:

• The register() / modify() methods only accept events as a bitmask of EVENT_READ | EVENT_WRITE
• The select() method only ever returns combinations of EVENT_READ | EVENT_WRITE

This design makes it impossible to use many platform-specific events provided by the underlying OS APIs (poll, epoll, kqueue). For example, https://github.com/python/cpython/issues/79698 describes a use case for EPOLLEXCLUSIVE on Linux. The biggest missed opportunity, however, is the inability to fully exploit kqueue() on BSD / macOS, which would allow async handling of regular files, signals, and others (see KQ_EVENT_*).

This limitation is relevant for asyncio, which uses the selectors module. For instance, on BSD / macOS, asyncio could be notified when a process terminates, enabling an optimization similar to https://github.com/python/cpython/pull/144047. Currenly asyncio on Linux already takes advantage of this approach via os.pidfd_open() + selectors.PollSelector (see source), but can't do the same for BSD / macOS because selectors.KqueueSelector doesn't allow registering custom select.kevent() objects.

Whereas it would be relatively easy to add a register_kevent() method, or to pass EPOLL* events directly to register(), it's less clear how the select() method should behave in this case, since it's apparently bound to the promise of returning either EVENT_READ or EVENT_WRITE.

Start with the selectors module API, especially KqueueSelector, PollSelector, register(), modify(), and select(), then read select.kevent and the asyncio use case in Lib/asyncio/unix_events.py around lines 857-891. Determine how platform-specific events should be registered and represented by select(); done means a settled API design that supports the cited OS events without breaking the existing read/write contract.