Add an `append_before_selection_mode` (subjet to bikeshedding)

[Diegovsky]

Hey so I find that extending the selection with a is an awesome feature and I miss it with i.

The reason I don't use a is because when appending at the EOL, it does a wraparound to the next line. If I then move the cursor to the penultimate character, appending adds it to the selection.

What I would like to propose is just another static command that enters append mode clearing the current selection.

Note: I searched a lot for issues regarding insert mode selection, append in-place not got no results

Also: changing i to ['append_mode', 'move_char_left'] exits append mode ;-;