Add support for custom billing plans

This commit includes updates to the billing schema to allow for the creation of custom billing plans. These types of plans have no line items associated with them. Changes have also been made to properly handle the absence of primary line items in such plans.

apps/web/tsconfig.json

@@ -26,5 +26,5 @@
     "lib/**/*.ts",
     "app"
   ],
-  "exclude": ["node_modules"]
+  "exclude": ["node_modules", ".next"]
 }