Fix Typescript 4.1 compile error

Typescript 4.1 will have [stricter rules for type predicates](microsoft/TypeScript#39258) like

```ts
export const isObject = (val: unknown): val is Record<any, any> =>
  val !== null && typeof val === 'object'
```

As a result of these rules, an expression in collectionHandlers.ts in
the reactivity package doesn't get narrowed to the type it did in TS 4.0
and below. Instead it gets an intersection type, which doesn't work with
subsequent code.

I restored the old type using a cast, since that's essentially what the
old version of Typescript was doing here.

Author: sandersn

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