add language HCL

Terraform doesn't register hcl and doesn't offer good DX if manually set
for editing e.g. nomad HCL files.

Incl. reformat with alejandra

modules/plugins/languages/default.nix

@@ -8,6 +8,7 @@ in {
     ./css.nix
     ./elixir.nix
     ./go.nix
+    ./hcl.nix
     ./html.nix
     ./java.nix
     ./lua.nix

modules/plugins/languages/hcl.nix

@@ -0,0 +1,71 @@
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}: let
+  inherit (lib.options) mkEnableOption mkOption;
+  inherit (lib.modules) mkIf mkMerge;
+  inherit (lib.types) package;
+  inherit (lib.nvim.types) mkGrammarOption;
+
+  cfg = config.vim.languages.hcl;
+in {
+  options.vim.languages.hcl = {
+    enable = mkEnableOption "HCL support";
+
+    treesitter = {
+      enable = mkEnableOption "HCL treesitter" // {default = config.vim.languages.enableTreesitter;};
+      package = mkGrammarOption pkgs "hcl";
+    };
+
+    lsp = {
+      enable = mkEnableOption "HCL LSP support (terraform-ls)" // {default = config.vim.languages.enableLSP;};
+      # TODO: it would be cooler to use vscode-extensions.hashicorp.hcl probably, shouldn't be too hard
+      # TODO: formatter, suppied by above or ...
+      # FIXME: or should we somehow integrate this:
+      #` https://git.mzte.de/nvim-plugins/null-ls.nvim/commit/e1fb7e2b2e4400835e23b9603a19813be119852b ??
+      package = mkOption {
+        description = "HCL ls package (terraform-ls)";
+        type = package;
+        default = pkgs.terraform-ls;
+      };
+    };
+  };
+  config = mkIf cfg.enable (mkMerge [
+    {
+      vim.pluginRC.hcl = ''
+        vim.api.nvim_create_autocmd("FileType", {
+          pattern = "hcl",
+          callback = function(opts)
+            local bo = vim.bo[opts.buf]
+            bo.tabstop = 2
+            bo.shiftwidth = 2
+            bo.softtabstop = 2
+          end
+        })
+
+        local ft = require('Comment.ft')
+        ft
+          .set('hcl', '#%s')
+      '';
+    }
+    (mkIf cfg.treesitter.enable {
+      vim.treesitter.enable = true;
+      vim.treesitter.grammars = [cfg.treesitter.package];
+    })
+
+    (mkIf cfg.lsp.enable {
+      vim.lsp.lspconfig.enable = true;
+      vim.lsp.lspconfig.sources = lib.optionalAttrs (! config.vim.languages.terraform.lsp.enable) {
+        terraform-ls = ''
+          lspconfig.terraformls.setup {
+            capabilities = capabilities,
+            on_attach=default_on_attach,
+            cmd = {"${cfg.lsp.package}/bin/terraform-ls", "serve"},
+          }
+        '';
+      };
+    })
+  ]);
+}