feat: extract editor used into own env var

Keep the old (and now default) behavior as fallback.

agenda

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+AGENDA_EDITOR=${VISUAL:-${EDITOR:-vi}}
+
 # The home of all files read and written by agenda.
 #
 # If this is left empty, this will default to the xdg user DOCUMENTS directory (in a
@@ -169,7 +171,7 @@ __edit() {
 
 	if [ "$opt_E" = "no" ]; then
 		export AGENDA_FILE="$file"
-		exec ${VISUAL:-${EDITOR:-vi}} "$file"
+		exec $AGENDA_EDITOR "$file"
 	else
 		echo "$file"
 	fi