Deprecate 'stick' feature of the 'smart_cursor'

[lambdalisue]

There is 'hide' feature so that I've decided to provide only that as 'hide_cursor'.

Rel: #307, #308, #312

If you'd like to handle cursor like Vim

Do nothing.
Remove let g:fern#disable_viewer_smart_cursor = 1 if you've used that

If you'd like to hide cursor

Add the following

let g:fern#hide_cursor = 1