Allow file picker changing root to parent directory or subdirectory

[L-Trump]

Helix is really an awesome editor, but it lacks the ability to navigate between structured files currently. Since it's not easy to implement a whole tree-view file navigator, some fancy workaround can be applied. With some keybindings in file picker view, reopen the file picker at the parent directory of current root or selected file. Thus we can at least perform simple navigate between directories, especially useful with file_picker_in_current_buffer_directory .

[kirawi]

See #11285