win32 window implementation in Rust

[bamidev]

Currently, there is a window implementation written for win32. It is used when using the browser framework Edge WebView2. It also used to be used in combination with CEF for Windows. But since the latter combination is pretty broken, it's really only used with Edge WebView2 now.
So it shouldn't be too invasive to move the win32 code into the rust side of things.

The benefit of this is that cross-compilation doesn't need the win32 header files to be available for the mingw compiler anymore. The other benefit would be that compilation for Edge WebView2 doesn't need to be done in a MSVC developer console necessarily.