LibJS: Add BigInt

Author: https://github.com/linusg
Commit: 0ff9d7e189
Pull-request: https://github.com/SerenityOS/serenity/pull/2509
Reviewed-by: https://github.com/alimpfard

Libraries/LibJS/Runtime/Object.h

@@ -105,6 +105,7 @@ public:
     virtual bool is_regexp_object() const { return false; }
     virtual bool is_string_object() const { return false; }
     virtual bool is_symbol_object() const { return false; }
+    virtual bool is_bigint_object() const { return false; }
 
     virtual const char* class_name() const override { return "Object"; }
     virtual void visit_children(Cell::Visitor&) override;