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

Cabal-syntax/src/Language/Haskell/Extension.hs

@@ -556,9 +556,6 @@ data KnownExtension
   | -- | Allow use of or-pattern syntax, condensing multiple patterns
     -- into a single one.
     OrPatterns
-  | -- | Along with 'ImplicitStagePersistence', this gives fine-grained control
-    -- over which modules are needed at each stage of execution.
-    ExplicitLevelImports
   | -- | Allow identifiers to be used at different levels than where they’re
     -- defined, using path-based persistence.
     ImplicitStagePersistence