Correct classDef and class grammar

Previously, you were allowed to define a class called 'default'
but were not allowed to use it because the classStatement grammar
expected an alphanum, which did not include the word DEFAULT

cypress/integration/rendering/flowchart.spec.js

@@ -891,4 +891,15 @@ graph TD
       { htmlLabels: true, flowchart: { htmlLabels: true }, securityLevel: 'loose' }
     );
   });
+  it('66: apply class called default on node called default', () => {
+    imgSnapshotTest(
+      `
+      graph TD
+        classDef default fill:#000,stroke:#000,stroke-width:4px,color:#fff 
+        default --> default2
+        class default,default2 default
+      `,
+      { htmlLabels: true, flowchart: { htmlLabels: true }, securityLevel: 'loose' }
+    );
+  });
 });