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.

Bắt đầu với API của mô-đun selectors, đặc biệt là KqueueSelector, PollSelector, register(), modify() và select(), sau đó đọc select.kevent và trường hợp sử dụng asyncio trong Lib/asyncio/unix_events.py quanh các dòng 857-891. Xác định cách các sự kiện dành riêng cho nền tảng nên được đăng ký và biểu diễn bởi select(); công việc được xem là hoàn tất khi có một thiết kế API đã thống nhất, hỗ trợ các sự kiện OS được nêu mà không phá vỡ hợp đồng read/write hiện có.