uv_backend_fd like support to make integration to OS gui event loop possible!

[congzhangzh]

I want to make webview_deno support async binding, which under the hood need pool tokio event in separate thread, and trigger on UI thread!

Is your feature request related to a problem? Please describe. no

Describe the solution you'd like A backend fd mechanism

Describe alternatives you've considered no

Additional context No

https://github.com/webview/webview_deno/issues/185#issuecomment-2501337626 https://docs.libuv.org/en/v1.x/loop.html#c.uv_backend_fd https://www.electronjs.org/blog/electron-internals-node-integration https://github.com/webview/webview/issues/578 https://github.com/webview/webview_deno/issues/185