feat: import and configure smartcolumn

modules/ui/smartcolumn/config.nix

@@ -0,0 +1,32 @@
+{
+  pkgs,
+  config,
+  lib,
+  ...
+}:
+with lib;
+with builtins; let
+  cfg = config.vim.ui.smartcolumn;
+in {
+  config = mkIf cfg.enable {
+    vim.startPlugins = [
+      "smartcolumn"
+    ];
+
+    vim.luaConfigRC.smartcolumn = nvim.dag.entryAnywhere ''
+      require("smartcolumn").setup({
+         colorcolumn = "${toString cfg.showColumnAt}",
+         disabled_filetypes = { ${concatStringsSep ", " (map (x: "\"" + x + "\"") cfg.disabledFiletypes)} },
+         -- { "help", "text", "markdown", "NvimTree", "alpha"},
+         custom_colorcolumn = {
+           -- TODO: use cfg.languages.<language>.columnAt when it's fixed to dynamically define per-language length
+           ruby = "120",
+           java = "120",
+           nix = "120",
+           markdown = "80",
+         },
+         scope = "file",
+      })
+    '';
+  };
+}