Embedding into existing app

Hi, first of all - thank you for the amazing project.

In my use scenario I'd like to embed `web-view` into existing C++ desktop app. E.g. C++ app will call extern Rust function and pass `HWND` or `NSView` to it and it will run my lib's Rust business logic and embed web-view into the window for which handlers was provided.

From my quick examination of the code it seems like it's possible to implement that. Do you have any plans supporting this functionality? I'm willing to help with a PR, but I'll probably will need some help from your side: I'm not familiar with cocoa at all and it would be great if you point me in the right direction with it.

Review the existing web-view window creation and handler entry points to determine how native window handles are currently obtained and managed. Compare the requested HWND and NSView embedding flows, then define the supported API and platform behavior before attempting implementation; the issue does not yet specify a concrete completion test.

Written by the indexing model from the issue text.