Application breaks when a file named `stdin` is in the exercises directory

[NicksPatties]

Describe the bug
If there is a file in the exercises directory called "stdin", the application will crash if it's randomly selected

To Reproduce
Steps to reproduce the behavior:

1. move your exercise files from the exercise directory elsewhere
2. echo "haha" >> ~/.config/sweet/exercises/stdin
3. sweet

Output
panic: runtime error: index out of range [1] with length 1

Expected behavior
I expect a file named stdin would not break the program

Sweet version
v0.2.0

Desktop (please complete the following information):

• OS: Pop_OS
• Version: 22.04 LTS