fix(compat): return value of vue compat set() (#9377)

According to https://v2.vuejs.org/v2/api/#Vue-set, Returns: the set value.

packages/runtime-core/src/compat/instance.ts

@@ -58,6 +58,7 @@ export interface LegacyPublicProperties {
 export function installCompatInstanceProperties(map: PublicPropertiesMap) {
   const set = (target: any, key: any, val: any) => {
     target[key] = val
+    return target[key]
   }
 
   const del = (target: any, key: any) => {