Add prefill of output filename

This cuts off the '.age' from the input, then lets
you choose the filename based on that

encrypting/rage.cheat

@@ -34,3 +34,4 @@ wl-paste | rage -d -i - "<input_file>" > "<output_file>"
 $ sapphic_public_age_key: echo age17r37eg8lmvsklve0rdajsts2rgca4j5twdmcglh6qufxj7qmzv0qywulhg
 
 $ input_file: find . -maxdepth 1 -type f | gum choose
+$ output_file: echo "<input_file>" | sed -e 's/\.age *$//' | gum input --header="Save file as.." --value=""