[nextgen/cardstack] big improvements

- Implement return types
- page numbers
- better subpage resolve delay
- error handling and system error cards
- code cleanup

labscore/cardstack/index.js

@@ -3,5 +3,6 @@ const { DynamicCardStack } = require("./DynamicCardStack");
 module.exports = {
   createDynamicCardStack: (context, options)=>{
     return new DynamicCardStack(context, options)
-  }
+  },
+  CARD_STACK_CONSTANTS: require("./constants"),
 }