Generate wrapper structs for unmanaged delegates

[JeremyKuhne]

Is your feature request related to a problem? Please describe.
APIs take native function pointers, which are defined in the metadata, but we have no wrapping projection for them.

For example, in the Win32Metadata:

[StructLayout(LayoutKind.Auto, CharSet = CharSet.Auto)]
[UnmanagedFunctionPointer(CallingConvention.Winapi)]
public delegate LRESULT WNDPROC([In] HWND param0, [In] uint param1, [In] WPARAM param2, [In] LPARAM param3);

Describe the solution you'd like

Create a "handle" struct like we do for HANDLE objects. Here is how I've wrapped this manually:

public unsafe readonly struct WNDPROC
{
    public readonly delegate* unmanaged<HWND, uint, WPARAM, LPARAM, LRESULT> Value;

    public WNDPROC(delegate* unmanaged<HWND, uint, WPARAM, LPARAM, LRESULT> value) => Value = value;
    public bool IsNull => Value is null;

    public static implicit operator WNDPROC(delegate* unmanaged<HWND, uint, WPARAM, LPARAM, LRESULT> value)
        => new(value);
    public static implicit operator delegate* unmanaged<HWND, uint, WPARAM, LPARAM, LRESULT>(WNDPROC value)
        => value.Value;
}

With that you should be able to use the struct in the APIs.