Courses endpoint returns incorrect format for difficulties

### Reproduction Steps

1. Create a course with two or more difficulties
2. Make an API call to `/wp-json/llms/v1/courses`

### Expected Behavior

Difficulties is an array of integers as per the documentation

### Actual Behavior

A string with a comma-separated list of integers is returned

### Tech Notes

We may want to create a `v2` of the endpoint with the fix so it won't break the code of anyone relying on the current incorrect format.

### This issue has been recreated:

+ [ ] Locally
+ [ ] On a staging site
+ [ ] On a production website
+ [ ] With only LifterLMS and a default theme