Add first-class support for winloop

[CNSeniorious000]

Summary

Winloop 0.3.0 just shipped with Python 3.14 support; I’ve tested it with some semi-large FastAPI apps on Windows and they ran fine. Since it claims to be in sync with uvloop, I believe it should be stable enough to serve as a drop-in replacement for asyncio. Additionally, having uvicorn use it directly could help uncover potential issues with winloop.

This PR adds first-class winloop support and includes it in the [standard] extras.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.