Support for [(@.length-1)]

[Zatte]

Hi,

How would one go about to get support for the [@.length - n] syntax illustrated @ https://goessner.net/articles/JsonPath/

I know that [start:stop] using negative indices is an ok workaround if we have control over the input. Given that gval and jsonpath is meant for end-users to input business logic it would be nice to expose full jsonpath engine and not jsonpath minus some features.

	v := interface{}(nil)
	json.Unmarshal([]byte(`{
		"welcome":{
				"message":["Good Morning", "Hello World!"]
			}
		}`), &v)
	welcome, err := jsonpath.Get("$.welcome.message[(@.length -1)]", v)
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
	fmt.Println(welcome)

https://go.dev/play/p/tDVmCrg6_OJ

=>

parsing error: $.welcome.message[(@.length -1)]	:1:29 - 1:30 unexpected "-" while scanning parentheses expected ")"

PS. Awesome work with both gval and jsonpath; Amazing libraries!