Add initial support to Windows

[enzo-santos]

Uses the OPENFILENAMEA structure of Windows API to open a "Save as" dialog on Windows, allowing the user to select a directory, where its path will be returned from saveAs.

This PR

• does assign the "Name" field of the dialog to the name and ext passed to saveAs
• does not (properly) assign the "Type" field of the dialog, represented by lpstrFilter on Windows API

• does not save the file inside the saveAs call. The user must still do

  final String? path = await FileSaver.instance.saveAs(bytes: bytes, /* ... */);
  if (path == null) return;
  
  await File(path).writeAsBytes(bytes);

Hope this helps in some way, since it's a MVP. Any contribution is welcome!