LibJS: Implement 'in' operator

Author: https://github.com/linusg
Commit: 11728b7db5
Pull-request: https://github.com/SerenityOS/serenity/pull/1931
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/AST.cpp

@@ -331,6 +331,8 @@ Value BinaryExpression::execute(Interpreter& interpreter) const
         return right_shift(interpreter, lhs_result, rhs_result);
     case BinaryOp::UnsignedRightShift:
         return unsigned_right_shift(interpreter, lhs_result, rhs_result);
+    case BinaryOp::In:
+        return in(interpreter, lhs_result, rhs_result);
     case BinaryOp::InstanceOf:
         return instance_of(interpreter, lhs_result, rhs_result);
     }
@@ -512,6 +514,9 @@ void BinaryExpression::dump(int indent) const
     case BinaryOp::UnsignedRightShift:
         op_string = ">>>";
         break;
+    case BinaryOp::In:
+        op_string = "in";
+        break;
     case BinaryOp::InstanceOf:
         op_string = "instanceof";
         break;