Feature Request: SHIFT_DELETE to Delete The Current Line

It is handy to use FZF to delete stale history items during walking through FZF hits with a shortcut, e.g. SHIFT_DELETE.

I personally use `history` to do such maintenance, but not convenient.

Start by tracing fzf's history mode and shortcut handling, then determine how a SHIFT_DELETE binding could remove the currently highlighted history item. Confirm whether the behavior must work across the mentioned shell contexts and define completion as deleting the selected stale history entry without disrupting navigation.

Written by the indexing model from the issue text.