plugin_registrar_windows.h was never fully updated for the Win32 switch,
and didn't actually compile. This introduces FlutterView (parallel to
the GLFW wrapper's FlutterWindow) as a ways of holding view-specific
functionality to expose via the plugin registrar, and moves HWND access
from the FlutterViewController to the FlutterView so that it's
available to plugins. This allows the implementation of plugins that need
access to the native HWND (e.g., moving or resizing the top-level window).

Adds simple unit tests of the new wrapper functionality, ensuring that the
files actually compile, and that the passthroughs work as expected.

This is a breaking change for Windows runners due to moving
GetNativeWindow() in the wrapper. It's not being done as a multi-stage
change (addition + deprecation + later removal) since this API is explicitly
unstable.