output: add extraPackagesAfter option

Allow users to add packages to the end of `PATH` in the neovim wrapper.
This is useful for LSP versions that might need to be overriden based on
the environment, e.g. `haskell-language-server` versions provided by a
project's devshell.

tests/test-sources/modules/output.nix

@@ -149,4 +149,16 @@
       end
     '';
   };
+
+  extraPackagesAfter =
+    { pkgs, ... }:
+    {
+      extraPackagesAfter = [ pkgs.hello ];
+
+      extraConfigLua = ''
+        if vim.fn.executable("hello") ~= 1 then
+          print("Unable to find hello package.")
+        end
+      '';
+    };
 }