Refactor _ckeditor5_get_langcode_mapping()

Problem/Motivation

Noticed in 📌 Pass RenderContext around in the Renderer Active but it turns out to be unrelated to that issue.

On a cold cache (tested with Umami), _ckeditor5_get_langcode_mapping()

We could move this to a service and then keep the mappings on a class property, then only fetch them from the cache once.

While individual cache gets aren't expensive, we do 472 cache gets on the Umami front page with a cold cache, and if each cache get is 0.3ms, that would take over 150ms.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet