LibCore: Introduce support for optional ArgsParser option values

Author: https://github.com/timschumi
Commit: a85b728768
Pull-request: https://github.com/SerenityOS/serenity/pull/14571
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibCore/ArgsParser.h

@@ -32,6 +32,7 @@ public:
 
     enum class OptionArgumentMode {
         None,
+        Optional,
         Required,
     };