Typescript: Clone maintains `readonly` property status

[MarcelWaldvogel]

An object (or object tree) to be cloned may have some properties marked as readonly to prevent accidental modification in a (shared) object.

In the clone, the properties should be modifiable in this use case. I also believe this is the main use case for readonly, so changing the return type should not introduce new problems.

(Alternatively, two different signatures could be provided; but I think this will needlessly complicate things.)

[trevor-vaughan]

I just hit this as well. Is there a potential of the fix getting merged?