Restructure: Group exercises by curriculum section

[mao-sz]

Because

Our JS exercises are no longer exclusive to the Foundations course. Having all exercises in the repo root would make it awkward to add new exercises in the middle. It makes sense to group exercises by curriculum section, just like the Ruby and CSS exercises repos.

This PR

• Moves all Foundations exercises into foundations/
• Moves all recursion exercises into computer_science/recursion/
• Renumbers all recursion exercises accordingly
• Amends README instructions for navigation
• Amends plop generators to allow for generating new exercises in nested dirs
• Fixes tests workflow to run only solution tests (Jest CLI uses regex not glob for test matching; was previously running all tests, not just solution ones)

Issue

Closes #533

Must only be merged alongside TheOdinProject/curriculum#30480 and TheOdinProject/curriculum#30502

Additional Information

A large diff but hopefully the diffs for each commit should be easier to review.

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. 01_helloWorld: Update test cases
• 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 any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder

[JoshDevHub]

Not going to pretend to be an expert in the file generation stuff here, but I trust you and everything else looks good. Helps that there are tests lol.

Let's do it.

[mao-sz]

I could look into adding tests for the plop stuff but that'd involve a fair amount of mocking FS/stdin/stdout stuff. Easiest is to checkout this PR and run npm run generate and play with a few options.