Add and configure gram-editor

Add nix-gram-extensions for building extensions as derivations.
Switch to own home-manager fork until wrapping of other editors is
possible upstream.

home/gram.nix

@@ -0,0 +1,65 @@
+{
+  config,
+  lib,
+  pkgs,
+  pkgs-unstable,
+  gram-extensions,
+  ...
+}:
+  let
+    extensions = with gram-extensions; [
+      typst
+      git-firefly
+    ];
+    extensions-dir = gram-extensions.linkGramExtensions extensions;
+  in {
+  programs.zed-editor = {
+    enable = true;
+    package = pkgs-unstable.gram;
+    extraPackages = [
+      pkgs.nil
+      pkgs.tinymist
+      pkgs.tofu-ls
+      pkgs.ltex-ls-plus
+    ];
+    userSettings = {
+      theme = {
+        mode = "system";
+        dark = "One Dark";
+        light = "One Light";
+      };
+      hour_format = "hour24";
+      vim_mode = true;
+      load_direnv = "shell_hook";
+
+      lsp = {
+        tinymist = {
+          settings = {
+            exportPdf = "onSave";
+            outputPath = "$root/$name";
+            preview.background.enabled = true;
+          };
+          initialization_options = {
+            preview.background.enabled = true;
+          };
+        };
+      };
+    };
+  };
+  home.file."Library/Application Support/Gram/extensions/installed" = {
+    enable = pkgs.stdenv.hostPlatform.isDarwin;
+    source = extensions-dir;
+    onChange = ''
+      cd "Library/Application Support/Gram/extensions"
+      mv index.json index.json.backup
+    '';
+  };
+  xdg.dataFile."gram/extensions/installed" = {
+    enable = pkgs.stdenv.hostPlatform.isLinux;
+    source = extensions-dir;
+    onChange = ''
+      cd "${config.xdg.dataHome}/gram/extensions"
+      mv index.json index.json.backup
+    '';
+  };
+}