Add input validation - the script only supports editing a single file

.local/bin/run0edit

@@ -54,6 +54,12 @@ clean_tempfiles() {
     rm --force "${tempfile}" "${tempfile}.orig"
 }
 
+if [ "$#" -ne "1" ]; then
+    echo "Expected exactly one input file. Usage:" 1>&2
+    echo "   $0 FILE" 1>&2
+    exit 1
+fi
+
 editor_cmd="$(default_editor)"
 echo "Editing using the command \"${editor_cmd}\""