feat(index-tracking): keep track of the `list._index` while navigating

[kimabrandt-flx]

This change tries to keep track of the list._index, by using an autocmd for BufEnter - which updates the index when entering a harpooned file - and setting the index for different functions inside the list.

There's room for improvements, but maybe it can help!?

This also removes the autocmd Filetype harpoon - which didn't seem to highlight the row?! - and replaces it with passing the index for setting the cursor to a row.

Maybe there should be an user-option, to enable this?!

This also merges #573, since I felt it's somehow related.

[mike-jl]

Works well for me. Would be nice, if it could get merged.