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' }
+    );
+  });
 });

packages/mermaid/src/diagrams/flowchart/parser/flow.jison

@@ -507,8 +507,8 @@ classDefStatement:CLASSDEF SPACE idString SPACE stylesOpt
     {$$ = $CLASSDEF;yy.addClass($idString,$stylesOpt);}
     ;
 
-classStatement:CLASS SPACE idString SPACE alphaNum
+classStatement:CLASS SPACE idString\[vertex] SPACE idString\[class]
-    {$$ = $CLASS;yy.setClass($idString, $alphaNum);}
+    {$$ = $CLASS;yy.setClass($vertex, $class);}
     ;
 
 clickStatement