LibJS: Add Value::is_integer()

Author: https://github.com/linusg
Commit: 8137f40b73
Pull-request: https://github.com/SerenityOS/serenity/pull/2159
Reviewed-by: https://github.com/alimpfard

Libraries/LibJS/Runtime/NumberConstructor.cpp

@@ -84,8 +84,7 @@ Value NumberConstructor::is_finite(Interpreter& interpreter)
 
 Value NumberConstructor::is_integer(Interpreter& interpreter)
 {
-    auto value = interpreter.argument(0);
-    return Value(value.is_number() && (i32)value.as_double() == value.as_double());
+    return Value(interpreter.argument(0).is_integer());
 }
 
 Value NumberConstructor::is_nan(Interpreter& interpreter)