types: fix Typescript 4.1 compile error (#2219)

close #2218

packages/reactivity/src/collectionHandlers.ts

@@ -21,7 +21,7 @@ const toReactive = <T extends unknown>(value: T): T =>
   isObject(value) ? reactive(value) : value
 
 const toReadonly = <T extends unknown>(value: T): T =>
-  isObject(value) ? readonly(value) : value
+  isObject(value) ? readonly(value as Record<any, any>) : value
 
 const toShallow = <T extends unknown>(value: T): T => value