Add move_prev_long_word_end command

[vwkd]

It seems there is no move_prev_long_word_end as counterpart to move_prev_long_word_start (B). Similarly, for select mode extend_prev_long_word_end as counterpart to extend_prev_long_word_start (B) doesn't exist.

In comparison, move_prev_word_end, albeit without default keybinding, exists as counterpart to move_prev_word_start (b).