modules/lsp: add onAttach option

Similar to `plugins.lsp.onAttach`, implement a "global" equivalent to
the per-server `on_attach` callback.

This is implemented using a `LspAttach` autoCmd.

modules/lsp/default.nix

@@ -20,6 +20,7 @@ in
   imports = [
     ./servers
     ./keymaps.nix
+    ./on-attach.nix
   ];
 
   config = {

modules/lsp/on-attach.nix

@@ -0,0 +1,48 @@
+{ lib, config, ... }:
+let
+  cfg = config.lsp;
+in
+{
+  options.lsp = {
+    onAttach = lib.mkOption {
+      type = lib.types.lines;
+      description = ''
+        Lines of lua to be run when a language server is attached.
+
+        > [!TIP]
+        > The variables `client` and `bufnr` are made available in scope.
+
+        This is a global equivialent to the per-server `on_attach` callback,
+        which can be defined via `lsp.servers.<name>.settings.on_attach`.
+
+        Unlike the per-server callback, which should be defined as a lua
+        callback function, this option should be defined as the function body.
+      '';
+      default = "";
+    };
+  };
+
+  config = lib.mkIf (cfg.onAttach != "") {
+    autoGroups.nixvim_lsp_on_attach.clear = false;
+
+    autoCmd = [
+      {
+        event = "LspAttach";
+        group = "nixvim_lsp_on_attach";
+        callback = lib.nixvim.mkRaw ''
+          function(args)
+            do
+              -- client and bufnr are supplied to the builtin `on_attach` callback,
+              -- so make them available in scope for our global `onAttach` impl
+              local client = vim.lsp.get_client_by_id(args.data.client_id)
+              local bufnr = args.bufnr
+
+              ${cfg.onAttach}
+            end
+          end
+        '';
+        desc = "Run LSP onAttach";
+      }
+    ];
+  };
+}