refactor(fish): read the non-trivial snippets from files

Move the interactiveShellInit block and the copy function body into
modules/fish/*.fish and pull them in with builtins.readFile, so the
non-trivial fish lives in real fish files (highlighting, fish_indent)
instead of inline Nix strings. The Auto-loader only collects .nix, so
the new .fish files are ignored by it. Rendered config is unchanged.

modules/fish/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