deprecate checkoutApi, avoid breaking

example/convex/example.ts

@@ -64,7 +64,7 @@ export const {
 
   // Cancels the current subscription.
   cancelCurrentSubscription,
-} = polar.api();
+} = polar.checkoutApi();
 
 // In a real app you'll set up authentication, we just use a
 // fake user for the example.

src/client/index.ts

@@ -301,6 +301,10 @@ export class Polar<
       }),
     };
   }
+  // Deprecated: use api() instead
+  checkoutApi() {
+    return this.api();
+  }
   registerRoutes(
     http: HttpRouter,
     {