ExtensionLifecycle should probably be an enum

Problem/Motivation

\Drupal\Core\Extension\ExtensionLifecycle is just a bunch of string-backed constants, and a static method to check if an arbitrary string matches one of these constant values. This sounds a lot like an enum.

Steps to reproduce

Proposed resolution

• Convert the class to an enum
• Remove ::isValid(), and use ::tryFrom() instead

Remaining tasks

Haven't really thought this through, maybe it's not a good idea. So figure out if it's worthwhile.

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet