Use an env var for fish shell integration

Less intrusive (in some ways) than adding symlinks to ~/.config/fish
Exploits the fact that fish loads scripts from XDG_DATA_DIRS on startup.
Thanks to @page-down for noticing

shell-integration/fish/vendor_completions.d/kitty.fish

@@ -0,0 +1,5 @@
+if functions -q _ksi_completions
+    complete -f -c kitty -a "(_ksi_completions)"
+else
+    complete -f -c kitty -a "(commandline -cop | kitty +complete fish)"
+end