Implement path via PathValue instead of Either[String, Int] for paths

[kyri-petrou]

This is more of an experiment just to see what we think.

So, what's wrong with Either[String, Int]? There are 2 things that are kind of wrong with it:

1. From a UX / dev perspective, It can be unclear on what it means unless you understand the structure of a graphql path
2. From a performance perspective, it causes unnecessary object allocations and boxing of Ints in the Executor.

With this PR, we substitute the Either[String, Int] with PathValue, a sealed trait that is extended by StringValue and IntValue.IntNumber. In a world where we've all migrated to Scala 3, this could have easily been a type PathValue = StringValue | IntValue.IntNumber but unfortunately we have to make the code compatible with Scala 2 🥲.

I also made a few changes to some of the methods used to parse paths / IntValues from String to avoid wrapping them in Try (which causes boxing) and avoid throwing exceptions as a control flow mechanism

[ghostdogpr]

Love it!