Recursion Exercises: Add new factorial exercise

[JoshDevHub]

Because

We want to bring the recursion exercises done in the recursion lesson in house.

Link the curriculum issue for this: TheOdinProject/curriculum#27265 (comment)

This PR

• Adds new exercise for calculating a factorial with recursion.

Issue

Closes #XXXXX

Additional Information

Leaving this as a draft for the time being. The main question I'd have is: are we okay with this organization / structure, or do we want to approach this differently?

Currently all the exercises in this repo are for "basic ruby." It might make sense to create an entirely new repo for these new exercises, which is the same way we do the custom enumerables project.

If we do leave them in this repo, is the way I have them organized here okay? Definitely interested in other maintainer input on the approach here.

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. Data types exercise: Update spec files
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR includes changes that needs to be updated on the solutions branch, I have created another PR (and linked it to this PR).

[JoshDevHub]

After discussing with the maintainer team, we agreed to go ahead with this organization, so I've opened this up for review.