tree-sitter: restore extraGrammars

pkgs/by-name/tr/tree-sitter/grammars/README.md

@@ -46,6 +46,31 @@ Attempt to build the new grammar: `nix-build -A tree-sitter-grammars.tree-sitter
 This will fail due to the invalid hash.
 Review the downloaded source, then update the source definition with the printed source `hash`.
 
+## Extending Grammars
+
+Overlays may extend the grammar set without modifying nixpkgs by overriding `tree-sitter` with `extraGrammars`.
+Extra grammars use the expanded `pkgs.tree-sitter.grammars` form: each key is the final `tree-sitter-*` attribute name, and each value provides `language`, `version`, and `src`.
+When added through an overlay, extra grammars are also available through `pkgs.tree-sitter.builtGrammars`, `pkgs.tree-sitter-grammars`, and `pkgs.tree-sitter.withPlugins`.
+
+```nix
+final: prev: {
+  tree-sitter = prev.tree-sitter.override {
+    extraGrammars = {
+      tree-sitter-foolang = {
+        language = "foolang";
+        version = "0.42.0";
+        src = final.fetchFromGitHub {
+          owner = "example";
+          repo = "tree-sitter-foolang";
+          tag = "v0.42.0";
+          hash = "";
+        };
+      };
+    };
+  };
+}
+```
+
 ## Pinning Grammar Sources
 
 To pin to a specific ref, append this to the source `url` to override the default version tag.

pkgs/by-name/tr/tree-sitter/package.nix

@@ -25,6 +25,7 @@
   enableStatic ? stdenv.hostPlatform.isStatic,
   wasmSupport ? false,
   webUISupport ? false,
+  extraGrammars ? { },
 }:
 
 let
@@ -52,18 +53,20 @@ let
 
     Use pkgs.tree-sitter.grammars.<name> to access.
   */
-  grammars = import ./grammars {
-    inherit
-      lib
-      nix-update-script
-      fetchFromGitHub
-      fetchFromGitLab
-      fetchFromSourcehut
-      fetchFromCodeberg
-      fetchpatch
-      stdenv
-      ;
-  };
+  grammars =
+    import ./grammars {
+      inherit
+        lib
+        nix-update-script
+        fetchFromGitHub
+        fetchFromGitLab
+        fetchFromSourcehut
+        fetchFromCodeberg
+        fetchpatch
+        stdenv
+        ;
+    }
+    // extraGrammars;
 
   /**
     Attrset of compiled grammars.