feat(file-manager): add mkdir subtask

[EternalRival]

While discussing path handling, I noticed that our file manager lacks the ability to create a directory.

This feature would be useful when testing an application on crosscheck.

Supposedly, our file manager is able to do all the basic operations except this one.

I haven't figured out what to do with the points, so I took some from Create empty file (probably not the best option)

[EternalRival]

@AlreadyBored