Run ghc-supported-extensions on CI

Author: sol

.github/workflows/validate.yml

@@ -262,6 +262,9 @@ jobs:
       # one possibly nightmarish final conditional. 'fail-fast' gets us
       # partway there, at least, but is still imperfect.
 
+      - name: Check that language extensions are registered for this GHC version
+        run: cabal run ghc-supported-extensions ghc
+
   validate-old-ghcs:
     name: Validate old ghcs ${{ matrix.extra-ghc }}
     # latest/latest doesn't have the old packages the old ghcs depend on