[Mappers] Mapper Not Used for Nested Types

**Describe the bug**

When using a `defaultMapper`, nested types are not mapped to their `ResolverType`. I've provided a CodeSandbox. I've added a field, `friends`, to `User`. `friends` is of type `UserFriendsConnection`.

I'd expect that the `defaultMapper`, in this case `Required<{T}>`, be applied to the nested `User` types of `UserFriendsEdge.node` and `UserFriendsConnection.nodes`.

```
export type UserFriendsConnection = {
__typename?: 'UserFriendsConnection',
edges: Array,
nodes: Array,
totalCount: Scalars['Int'],
};

export type UserFriendsEdge = {
__typename?: 'UserFriendsEdge',
cursor: Scalars['String'],
node: User,
};
```

**To Reproduce**
Steps to reproduce the behavior:

https://codesandbox.io/s/graphql-codegen-issue-template-o0y03