fish-foreign-env: move to fishPlugins.foreign-env

And relocate the installed fish functions to the `vendor_functions.d` so
that they're automatically loaded.

pkgs/shells/fish/plugins/default.nix

@@ -6,6 +6,8 @@ lib.makeScope newScope (self: with self; {
 
   fishtape = callPackage ./fishtape.nix { };
 
+  foreign-env = callPackage ./foreign-env { };
+
   pure = callPackage ./pure.nix { };
 
 })