Rename kitty-tool to kitten

It's easier to type, and cuter.

Also, most, if not all of the TUI parts of
kitty's kittens will eventually be re-written into kitten.

The only downside I can see is that we cant tab complete kitty
anymore, but hopefully there will be less reason to run kitty
from the shell as command line tools migrate to kitten.

Meowrrrr!!!

kittens/clipboard/main.py

@@ -78,7 +78,7 @@ the clipboard. Some examples:
 
 usage = '[files to copy to/from]'
 if __name__ == '__main__':
-    raise SystemExit('This should be run as kitty-tool clipboard')
+    raise SystemExit('This should be run as kitten clipboard')
 elif __name__ == '__doc__':
     from kitty.cli import CompletionSpec
     cd = sys.cli_docs  # type: ignore