Add `EventLoopProxy::waker(self) -> Waker`

[madsmtm]

As part of https://github.com/rust-windowing/winit/issues/3367, we're considering removing the generic on Event<T>, EventLoop<T>, EventLoopProxy<T> and the callback. By extension, we'd be getting rid of Event::UserEvent(T).

One approach to doing so would be to allow converting EventLoopProxy to the standard library's Waker type, which allows the user to wake a task (in this case the event loop) from any thread; then they can implement what they need themselves on top of that, using the standard library's mpsc::channel, mpsc::sync_channel or likewise, depending on their needs.

This is likely not a perfect solution for wakeups, we still need to properly figure out how to do timers; but this can get us some of the way there.

Blocked on https://github.com/rust-windowing/winit/pull/3449, we need to ensure that Sync is actually sound.

CC @notgull, I think this may be slightly useful for async?

Questions:

• [ ] Do we need a UserWoken event, or is NewEvents is enough?
• [x] Should we hide the Waker behind a custom type like EventLoopWaker, in case we want to expose further functionality in the future?
  • We keep EventLoopProxy, and allow that to be converted to Waker.
• [ ] Is Waker too heavy-weight for this? Can we just use a function wake(&self) on EventLoopProxy? Or maybe both?
• [ ] Does the user need the ability to handle the case where the event loop has been destroyed? Or is that handled fine enough through other means, like in mpsc?

TODO:

• [ ] Finish implementation on all platforms.
• [ ] Properly test Waker implementations with updated example.
• [ ] Add an entry to CHANGELOG.md.
• [ ] Update documentation.