LibJS: Add Number.isSafeInteger()

Author: https://github.com/linusg
Commit: 5ecc7e6a1c
Pull-request: https://github.com/SerenityOS/serenity/pull/1687

Libraries/LibJS/Runtime/NumberConstructor.h

@@ -41,6 +41,8 @@ public:
 private:
     virtual bool has_constructor() const override { return true; }
     virtual const char* class_name() const override { return "NumberConstructor"; }
+
+    static Value is_safe_integer(Interpreter&);
 };
 
 }