Remove need for EventLoopProxy to be NonSend (#14198)

# Objective

- Continue to pare down the uses on NonSend resources in the engine. In
this case, EventLoopProxy used to be `!Sync`, but is now `Sync` in the
latest version of winit.

## Solution

- New type `EventLoopProxy` as `EventLoopProxyWrapper` to make it into a
normal resource.
- Update the `custom_user_event` example as it no longer needs to
indirectly access the `EventLoopProxy` through a static variable
anymore.

## Testing

- Ran the example. The resource exists just for users to use, so there
aren't any in engine uses for it currently.

---

## Changelog

- make EventLoopProxy into a regular resource. 

## Migration Guide

`EventLoopProxy` has been renamed to `EventLoopProxyWrapper` and is now
`Send`, making it an ordinary resource.

Before:
```rust
event_loop_system(event_loop: NonSend<EventLoopProxy<MyEvent>>) {
    event_loop.send_event(MyEvent);
}
```

After:
```rust
event_loop_system(event_loop: Res<EventLoopProxy<MyEvent>>) {
    event_loop.send_event(MyEvent);
}
```