openapi-types: support x- specification extensions everywhere without a pre-existing index signature

[mitchell-merry]

Fixes #768

Per the Swagger 2.0, OpenAPI 3.0, and OpenAPI 3.1 specs, most objects should support ^x- properties for the purposes of extending the specification. However, openapi-types does not support these yet.

This PR introduces the Extensible interface, which is used to specify the types that should support this. It also bumps the prettier package to 2.2.0, which is the earliest version which supports TypeScript templating strings.

As is, this PR contains breaking changes to the types, to better fit the specfication! See details in the comments.

Interested to hear your thoughts!