refactor(fish): lay the Module out like ~/.config/fish

Move the Module to modules/fish/fish.nix and mirror a real fish config
directory beside it: config.fish (interactive init), conf.d/done.fish
(the done plugin tuning, now a real conf.d file), and functions/copy.fish.
completions/ and themes/ are omitted as they have no content and git
cannot track empty directories.

modules/fish/functions/copy.fish

@@ -0,0 +1,8 @@
+set count (count $argv | tr -d \n)
+if test "$count" = 2; and test -d "$argv[1]"
+    set from (echo $argv[1] | trim-right /)
+    set to (echo $argv[2])
+    command cp -r $from $to
+else
+    command cp $argv
+end