modules/performance: add plenary filetypes directory to default pathsToLink

plenary.nvim is often pulled as a dependency of other plugins.
It has filetype definitions in `data/plenary/filetypes` directory.
Even though I don't think there are plugins using it instead of
vim.filetype, but it should be no harm to add this directory by default.

modules/performance.nix

@@ -56,6 +56,8 @@ in
       "/after"
       # ftdetect
       "/ftdetect"
+      # plenary.nvim
+      "/data/plenary/filetypes"
     ];
   };
 }

tests/test-sources/modules/performance/combine-plugins.nix

@@ -374,4 +374,14 @@ in
         }
       ];
     };
+
+  # Test if plenary.filetype is working
+  plenary-nvim = {
+    performance.combinePlugins.enable = true;
+    extraPlugins = [ pkgs.vimPlugins.plenary-nvim ];
+    extraConfigLuaPost = ''
+      -- Plenary filetype detection is usable
+      assert(require("plenary.filetype").detect(".bashrc") == "sh", "plenary.filetype is not working")
+    '';
+  };
 }