Make arrays behave like lists in reflection

[themasch]

Objective

Currently, arrays cannot indexed using the reflection path API. This change makes them behave like lists so x.get_path("list[0]") will behave the same way, whether x.list is a "List" (e.g. a Vec) or an array.

Solution

When syntax is encounterd [ <idx> ] we check if the referenced type is either a ReflectRef::List or ReflectRef::Array (or ReflectMut for the mutable case). Since both provide the identical API for accessing entries, we do the same for both, although it requires code duplication as far as I can tell.

This was born from working on #5764, but since this seems to be an easier fix (and I am not sure if I can actually solve #5812) I figured it might be worth to split this out.