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

plugin/projectionist.lua

@@ -0,0 +1,57 @@
+vim.g.projectionist_heuristics = {
+  ["composer.json"] = {
+    ["src/*.php"] = {
+      type = "source",
+      alternate = "tests/{}Test.php",
+    },
+
+    ["src/Controller/*.php"] = {
+      type = "controller",
+    },
+
+    ["src/Model/*.php"] = {
+      type = "model",
+    },
+
+    ["src/Service/*.php"] = {
+      type = "service",
+    },
+
+    ["tests/*Test.php"] = {
+      type = "test",
+      alternate = "src/{}.php",
+    },
+  },
+
+  ["fractal.config.js"] = {
+    ["*.config.yml"] = {
+      type = "config",
+      alternate = "{dirname}/{basename}.twig",
+    },
+
+    ["*.twig"] = {
+      type = "template",
+      alternate = "{dirname}/{basename}.config.yml",
+    },
+  },
+
+  ["package.json"] = {
+    ["src/*.ts"] = {
+      type = "source",
+      alternate = "tests/{}.test.ts",
+    },
+
+    ["src/*.service.ts"] = {
+      type = "source",
+      alternate = "tests/{}.test.ts",
+    },
+
+    ["tests/*.test.ts"] = {
+      type = "test",
+      alternate = "src/{}.ts",
+    },
+  },
+}
+
+-- Switch to an alternative file.
+vim.keymap.set("n", "<leader>aa", "<cmd>A<cr>")