Breaking change with .extend()

The docs fail miserably at explaining how to extend Joi.

When you google how to create extensions for Joi there are hundreds of posts and libraries that have code just like the one posted in [this issue](https://github.com/sideway/joi/issues/2453#issue-684440919).

This one in particular [@meanie/joi-country](https://github.com/meanie/joi-country/blob/master/index.js) has `"Joi": "^17.0.0 "` in it's package.json but the library doesn't even work with the current version `17.4.2`.

Here's a codesanbox showing how it fails with `.country is not a function`: https://codesandbox.io/s/amazing-cerf-hwjo0?file=/src/index.js

Was there a breaking change without a major semver increase?