PHP 8.1 supports Enums, and [Enums follow class-semantics](https://php.watch/versions/8.1/enums#class-semantics-autoload).
Composer's class-map generator currently looks for `class`, `interface`, and `trait` keywords. If Composer is run in PHP 8.1 or later, Composer now additionally looks for `enum` keyword as well. This is similar to how Hack's `enum` support is added.
This PR also adds tests for basic enums, backed enums, namespaced enums, and an enum that implements an interface and extends a class.
228428747a