LibCore: Extract Stream::OpenMode conversion helper into static function

Author: https://github.com/kleinesfilmroellchen
Commit: 1b67e19bd6
Pull-request: https://github.com/SerenityOS/serenity/pull/13603
Issue: https://github.com/SerenityOS/serenity/issues/13555
Reviewed-by: https://github.com/BertalanD
Reviewed-by: https://github.com/alichraghi ✅
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibCore/Stream.h

@@ -205,6 +205,8 @@ public:
 
     virtual ~File() override { close(); }
 
+    static int open_mode_to_options(OpenMode mode);
+
 private:
     File(OpenMode mode)
         : m_mode(mode)