fix(typings): incompatible type issue for nested strict object schemas

[LeoAso]

The type issue that prevented strict object schemas from being nested has been resolved. The following is no longer incorrectly flagged as an error.

interface ObjType {
  firstName: string;
  lastName: string;
  nestedObject: {
    middleName: string;
  }
}

// no errors
Joi.object<ObjType, true>({
  firstName: Joi.string(),
  lastName: Joi.string(),
  nestedObject: Joi.object<ObjType['nestedObject'], true>({
    middleName: Joi.string()
  })
})

This PR fixes #2764.

[phillip-kil]

@LeoAso What is the state of this? We are heavily relying on this and have a lot of workarounds we'd like to get rid of :)

[LeoAso]

@phillip-kil It seems the maintainers haven't gotten around to reviewing it yet.

[Marsup]

This looks fine, although, would you mind adding tests in this file to avoid any further regression?

[Marsup]

Superseded by #2836, thanks for doing the initial work though!