Add new practice exercise: swift-scheduling

[jimmytty]

I have set difficulty to 5 (Medium) (but it can be 8 actually).

Out of curiosity, I only found 7 tracks that implemented this exercise and only 1 set the difficulty to easy:

| track      | difficulty   |
|------------+--------------|
| csharp     |            6 |
| elixir     |            6 |
| elm        |            7 |
| java       |            4 |
| powershell |            3 |
| python     |            4 |
| tcl        |            4 |

[IsaacG]

This is one of the more recently added exercises to the problem spec so it's not yet widely implemented.