Remove EloquentItem and NullItem and rename JenssegersItem

[JaZo]

Description

I removed EloquentItem and NullItem and renamed/moved Items\JenssegersItem to Item.

Motivation and context

The EloquentItem was experimental, but it had some limitations which could not be fixed without being too opinionated. Besides, no one was using it. Because of this, I decided to remove it. NullItem was used only internally and I refactored it to null. This left us with only JenssegersItem so I renamed/moved that to just Item to clean up. In the process I changed some classes that depended on the JenssegersItem instead of the ItemInterface.

How has this been tested?

Run unit tests to confirm everything still works.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.