macos: add a separate alternative macro for non-translation marker use

essentially the macos side of 6cd08480,
as requested in !1172 discussion.

the new `NSTR()` macro is the equivalent of `qfut()`. it is intended for
use instead of `_NS()` when wrapping a define name, variable or function
call, where the system that scans for strings we want to be translated
should not apply, leaving `_NS()` to just be used for marking such strings.

i have avoided renaming `_NS()` to avoid unnecessary widespread change.

modules/gui/macosx/extensions/NSString+Helpers.h

@@ -28,7 +28,12 @@
 #import <Cocoa/Cocoa.h>
 #import <vlc_input.h>
 
-#define _NS(s) ((s) ? toNSStr(vlc_gettext(s)) : @"")
+#define NSTR(s) ((s) ? toNSStr(vlc_gettext(s)) : @"")
+
+/**
+ * For marking translatable static strings (like `_()`)
+ */
+#define _NS(s) NSTR(s)
 
 /**
  * Get a contextualized translation string