DeepReadonly type

[stefnotch]

What problem does this feature solve?

Currently, readonly({}) uses the Readonly<> type. This makes all properties readonly. However, it should also affect nested properties.

any nested property accessed will be readonly as well
-- https://vue-composition-api-rfc.netlify.app/api.html#readonly

So, I suggest adding a DeepReadonly type like the one in ts-essentials and using it for the readonly({}) function. This would result in Typescript being able to catch more errors at compile time.

What does the proposed API look like?

export type DeepReadonly<T> = ...;

export declare function readonly<T extends object>(target: T): DeepReadonly<UnwrapNestedRefs<T>>;