Remove Illuminate dependencies

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This removes the internal dependencies on the Illuminate helpers assumed to be provided by Sage 9/10. With this change, Navi should now be usable in any WordPress environment. I've done some minimal testing on an existing project, but more testing would certainly be wise.

This would represent a ⚠️ breaking change ⚠️ if implemented: It strips out all Fluent functionality, so any user implementation depending on Fluent methods will immediately explode. If this is a real no-go, then I can look at just manually copying over the classes, etc, since we want to avoid any dependencies (and thus dependency hell w/ relatively popular packages). I left that out in this initial PR for the following reasons:

1. Navi is light, and I'd like to see it stay light
2. The use case(s) described in the Readme are still supported with this version (most of them involve getting an array out of Navi ultimately anyway and now it just uses arrays internally instead of collections)
3. It would feel gross to do that

Also of note:
I've added a method called getItems() which returns an array of items (i.e. you'd use it as toArray() is used in the original examples). I did this because toArray() makes no semantic sense when all internal data is now arrays. However I have added a toArray() method as well that just returns getItems() to make the transition from older user implementations a little easier.

Let me know if you'd like me to make any changes or take it in a different direction.