refactor: explicitly convert HANDLE to intptr_t for _open_osfhandle()

Problem:
The first parameter of "_open_osfhandle()" is an intptr_t; however, a HANDLE is passed.
The official documentation of [_open_osfhandle](https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/open-osfhandle) suggests to cast the HANDLE to intptr_t.
MSVC/cl is able to perform an implicit type cast.
However, LLVM/clang-cl will generate an compilation error.

Solution:
Explicitly convert HANDLE to intptr_t for _open_osfhandle().

src/nvim/fileio.c

@@ -1683,7 +1683,7 @@ failed:
       HANDLE conin = CreateFile("CONIN$", GENERIC_READ | GENERIC_WRITE,
                                 FILE_SHARE_READ, (LPSECURITY_ATTRIBUTES)NULL,
                                 OPEN_EXISTING, 0, (HANDLE)NULL);
-      vim_ignored = _open_osfhandle(conin, _O_RDONLY);
+      vim_ignored = _open_osfhandle((intptr_t)conin, _O_RDONLY);
 #endif
     }
   }