Add permutations exercise for recursion

[JoshDevHub]

Because

I'm working on adding Recursion Exercises to this repo because we determined it'd be beneficial to have this content in-house. See the linked issue for more information.

This PR

• Adds an exercise, tests, and a solution for a #permutations method. This method takes an array of unique integers and returns a nested array containing all of its permutations.

Issue

Related to #114

Additional Information

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 addresses an open issue, it is linked in the Issue section
• If this PR includes changes to exercises, they are also changed in the solutions folder.