refactor: move some anonymous enums back to non-defs headers (#26622)

It isn't really useful to put anonymous enums only used as arguments to
functions calls in _defs.h headers, as they will only be used by a file
that calls those functions, which requires including a non-defs header.

Also move os_msg() and os_errmsg() back to message.h, as on Windows they
are actual functions instead of macros.

Also remove gettext.h and globals.h from private/helpers.h.

src/nvim/os/lang.c

@@ -20,6 +20,7 @@
 #include "nvim/ex_cmds_defs.h"
 #include "nvim/garray.h"
 #include "nvim/gettext.h"
+#include "nvim/globals.h"
 #include "nvim/macros_defs.h"
 #include "nvim/memory.h"
 #include "nvim/message.h"