feat: add file following support to the `files` section

[GustavoKatel]

re #38

This is a work in progress!

TODO list:

• [x] Add option to move the cursor to the sidebar when calling focus directly
• [x] When we try jump to a file in a directory that is not open in the tree, it silently ignores, but we should instead open the parents and jump to that file. See "TODOs" in the code
• [ ] Add docs