Validate RBI config in CI (#228)

.github/workflows/ci.yml

@@ -35,6 +35,12 @@ jobs:
         with:
           ruby-version: 3.2
           bundler-cache: true
+      # This is just to ensure the default config is valid. The target is intentionally set
+      # to a directory that doesn't contain any .rb or .rbi files.
+      - name: Validate default config
+        run: |
+          bin/rubocop --config config/rbi.yml config
+          bin/rubocop --config config/default.yml config -r rubocop-sorbet
       - name: Lint Ruby files
         run: bin/rubocop
       - name: Verify documentation is up to date