win: Fix reading content from stdin (#8267)

Fixes #6890 by reading from the Windows console input buffer after
stdin has been closed.

Vim defines HAVE_DUP for Windows and does the close-dup dance[1]:

    close(0);
    dup(2);

which always fails, then falls back to reading from the Windows console
input buffer[2].

[1] e7499ddc33/src/fileio.c (L2397-L2398)
[2] e7499ddc33/src/os_win32.c (L1703-L1714)

src/nvim/fileio.c

@@ -1737,9 +1737,17 @@ failed:
   xfree(buffer);
 
   if (read_stdin) {
-    /* Use stderr for stdin, makes shell commands work. */
     close(0);
+#ifndef WIN32
+    // On Unix, use stderr for stdin, makes shell commands work.
     ignored = dup(2);
+#else
+    // On Windows, use the console input handle for stdin.
+    HANDLE conin = CreateFile("CONIN$", GENERIC_READ | GENERIC_WRITE,
+                              FILE_SHARE_READ, (LPSECURITY_ATTRIBUTES)NULL,
+                              OPEN_EXISTING, 0, (HANDLE)NULL);
+    ignored = _open_osfhandle(conin, _O_RDONLY);
+#endif
   }
 
   if (tmpname != NULL) {