LibJS: Add Math.pow()

Author: https://github.com/linusg
Commit: 97366f4dd4
Pull-request: https://github.com/SerenityOS/serenity/pull/1931
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Runtime/MathObject.h

@@ -50,6 +50,7 @@ private:
     static Value sin(Interpreter&);
     static Value cos(Interpreter&);
     static Value tan(Interpreter&);
+    static Value pow(Interpreter&);
 };
 
 }