Generator pattern cache grows unbounded in long-running process

Problem/Motivation

In long-running processes (such as migrations as of https://www.drupal.org/project/drupal/issues/3006750), the unbounded growth of pathauto.generator::$patterns contributes to memory exhaustion.

Steps to reproduce

Run a large enough migration with a small-enough configured memory, which generates aliases, and see that pathauto.generator::$patterns becomes arbitrarily large before the process dies due to exhausting memory.

Proposed resolution

Rework to make use of the new in-memory LRU cache to establish an upper bound on the number of items held in memory.

Remaining tasks

User interface changes

API changes

Data model changes