[feat] nvim: add nvim-silicon plugin

Also reorganize out of tree plugins in preparation for
`vim-plugin-updater` integration.

nvim/extraPlugins/default.nix

@@ -0,0 +1,4 @@
+{pkgs}: {
+  vim-jjdescription = pkgs.callPackage ./vim-jjdescription.nix {};
+  nvim-silicon = pkgs.callPackage ./nvim-silicon.nix {};
+}

nvim/extraPlugins/nvim-silicon.nix

@@ -0,0 +1,15 @@
+{
+  vimUtils,
+  fetchFromGitHub,
+  silicon,
+}:
+vimUtils.buildVimPlugin {
+  name = "nvim-silicon";
+  dependencies = [silicon];
+  src = fetchFromGitHub {
+    owner = "michaelrommel";
+    repo = "nvim-silicon";
+    rev = "v1.0.0";
+    hash = "sha256-cZOzgzLUNC9qOS2m/rc6YJfpNGdRTSCAdEPQDy+wT6I=";
+  };
+}

nvim/nixvim.nix

@@ -55,11 +55,12 @@ in {
       inherit (import ./augroups.nix) autoGroups autoCmd;
       extraPlugins = let
         plugins = pkgs.unstable.vimPlugins;
-        jjdescription = pkgs.unstable.callPackage ./vim-jjdescription.nix {};
+        extraPlugins = import ./extraPlugins {pkgs = pkgs.unstable;};
       in [
         plugins.nui-nvim
         plugins.nvim-web-devicons
-        jjdescription
+        extraPlugins.vim-jjdescription
+        extraPlugins.nvim-silicon
       ];
       # Formatting & linters
       extraPackages = [