Add languages extension groups to grammar

.github/workflows/quick-jobs.yml

@@ -141,8 +141,10 @@ jobs:
           restore-keys: linux-store-buildinfo-doc-diff
       - name: Build buildinfo-reference-generator
         run: ${{ env.cabal_build }}
-      - name: Are buildinfo docs up to date?
-        run: make doc/buildinfo-fields-reference.rst
+      - name: Are cabal package syntax docs up to date?
+        run: make doc/cabal-package-syntax.rst
+      - name: Are language extensions docs up to date?
+        run: make doc/language-extensions.rst
       - name: Cache dependencies
         uses: actions/cache/save@v4
         if: always() && steps.cache.outputs.cache-hit != 'true'

Cabal-described/Cabal-described.cabal

@@ -23,5 +23,6 @@ library
 
   exposed-modules:
     Distribution.Described
+    Distribution.Described.Extension
     Distribution.Utils.CharSet
     Distribution.Utils.GrammarRegex

Cabal-described/src/Distribution/Described.hs

@@ -400,9 +400,6 @@ instance Described ExeDependency where
 instance Described ExposedModule where
     describe _ = RETodo
 
-instance Described Extension where
-    describe _ = RETodo
-
 instance Described FlagAssignment where
     describe _ = REMunch RESpaces1 $
         REUnion [fromString "+", fromString "-"] <> describe (Proxy :: Proxy FlagName)