zsh:keys:rationalize_dots: Add .. scenario

Handle an LBUFFER value of `..` before pressing `.` by adding `/../`.
This scenario can only occur by deleting previously added segments or
using the new `default_dot` (See previous commit 72a8cfbcb ("zsh:keys:
Add normal `.` behaviour on `Ctrl-.`")).

.config/zsh/zshrc.d/60-keys.zsh

@@ -86,6 +86,10 @@ function rationalize_dots {
 	# Rationalize dots at BOL or after a space or slash.
 	if [[ "$LBUFFER" =~ "(^|[ /])\.\./$" ]]; then
 		LBUFFER+=../
+	elif [[ "$LBUFFER" =~ "(^|[ /])\.\.$" ]]; then
+		# NOTE: This scenario occurs only if backspace or `default_dot` was
+		#       used.
+		LBUFFER+=/../
 	elif [[ "$LBUFFER" =~ "(^|[ /])\.$" ]]; then
 		LBUFFER+=./
 	else