Add key combinations for ctrl-delete and shift-delete (#3284)

Currently there is not option to bind ctrl-delete and shift-delete. As
suggested by issue #3240, shift-delete could be used to bind "delete
entry from history" as it is a common way to do so in other
applications, e.g. browsers.

This, however, does only implement to use the key combination itself and
does not assign a default action to any of them. This does enable to
call one's all predefined actions. With the exec action this can
expanded like the issue #3240 suggested.
If desirable, the key combinations could later get a default behavior.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

src/tui/tui.go

@@ -41,6 +41,7 @@ const (
 	CtrlZ
 	ESC
 	CtrlSpace
+	CtrlDelete
 
 	// https://apple.stackexchange.com/questions/24261/how-do-i-send-c-that-is-control-slash-to-the-terminal
 	CtrlBackSlash
@@ -74,6 +75,7 @@ const (
 	SDown
 	SLeft
 	SRight
+	SDelete
 
 	F1
 	F2