CKEditor 5 and Drupal 10 readiness

Problem/Motivation

In Drupal 10, CKEditor 4 will be replaced with CKEditor 5. It can be tested following the steps on https://www.drupal.org/docs/core-modules-and-themes/core-modules/ckeditor-5-module/testing-the-ckeditor-4-to-5-upgrade-path

GeshiFilter module provides a plugin for CKEditor to add syntaxically colorated code blocks to a text field, and there is no upgrade path for that:
The CKEditor 4 button GeshiFilter does not have a known upgrade path. If it allowed editing markup, then you can do so now through the Source Editing functionality.

Steps to reproduce

Enable GeshiFilter module
Enable CKEditor 5 module.
Configure the Basic HTML format by adding the GeshiFilter button, and save it.
Configure it again by selecting "CKEditor 5" instead of "CKEditor": the above message is displayed.

Proposed resolution

A CKEditor 5 plugin is needed. It can be declared via a Drupal specific plugin class and configured via a *.ckeditor5.yml file.

I suggest to mimic the core codeblock CKEditor 5 plugin added from 9.4.x as a codebase to build on GeshiFilter upgrade.