Handle structs incorrectly using nint field instead of IntPtr

[AArnott]

Actual behavior

A recent regression has led to typedef structs that represent handles using nint everywhere instead of the more opaque IntPtr type.

Expected behavior

Use IntPtr for opaque, pointer-sized value (e.g. IntPtr handle)
Reserve nint for integers that code is expected to manipulate and interpret (e.g. nint Length)

Repro steps

1. NativeMethods.txt content:

HWND

2. NativeMethods.json content (if present):
   N/A

3. Any of your own code that should be shared?

Context

• CsWin32 version: 0.1.506-beta
• Win32Metadata version (if explicitly set by project):
• Target Framework: [e.g. netstandard2.0]
• LangVersion (if explicitly set by project): [e.g. 9]

[AArnott]

Actually this only impacted a subset of handle types that don't use the RAIIFreeAttribute on the typedefs. We already special cased HGDIOBJ. I guess we'll also special case HWND.