Fix fn serialization with older versions of integration (#354)

Author: mattcompiles

.changeset/strange-cooks-trade.md

@@ -0,0 +1,5 @@
+---
+'@vanilla-extract/css': patch
+---
+
+Fix function serialization with older versions of the `@vanilla-extract/integration` package

packages/css/src/functionSerializer.ts

@@ -16,7 +16,9 @@ export function addFunctionSerializer<Target extends object>(
   target: Target,
   recipe: SerializerConfig,
 ) {
-  Object.defineProperty(target, '__function_serializer__', {
+  // TODO: Update to "__function_serializer__" in future.
+  // __recipe__ is the backwards compatible name
+  Object.defineProperty(target, '__recipe__', {
     value: recipe,
     writable: false,
   });