Make Windows main thread checks more forgiving

[PJB3005]

The check on Windows to enforce that the event loop is created from the main thread is not fully reliable (https://github.com/rust-windowing/winit/issues/3999). Given that this check is not necessary on Windows and only exists more as an assert to enforce platform consistency in development, I have made it more lax:

• It now produces a tracing::warn!() message instead of an outright panic.
• The check is only compiled in with cfg(debug_assertions)

Fixes #3999

• [x] Tested on all platforms changed
• [x] Added an entry to the changelog module if knowledge of this change could be valuable to users
• [x] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [x] Created or updated an example program if it would help users understand this functionality
• [x] Updated feature matrix, if new features were added or implemented

[PJB3005]

Any idea how we'd go about checking that?

[PJB3005]

Alright, I figured out a way to check it, hope this is sufficient.

[notgull]

I would rather use a better way to detect if we are on the main thread, if possible. I've opened #4006 for this. Let me know if it works.