Re-add vim-projectionist

Whilst I wasn't able to get it to work for all my use cases previously,
it is useful for creating alternative files in Fractal, such as
switching between the `*.config.yml` and `*.twig` files for a component.

Other use cases, such as creating alternative files for classes and
tests in PHP and Drupal projects, I can look at again now the plugin is
installed.

lib/default.nix

@@ -70,6 +70,7 @@ rec {
       vimPlugins.vim-highlightedyank
       vimPlugins.vim-just
       vimPlugins.vim-nix
+      vimPlugins.vim-projectionist
       vimPlugins.nvim-spectre
       vimPlugins.vim-obsession
       vimPlugins.vim-pasta