Add new defacto Mentoring Coordinators to MAINTAINERS.txt

Problem/Motivation

James Shield 'lostcarpark' has been actively coordinating mentoring in Europe for a couple of years.

Proposed resolution

Add de facto core mentoring coordinators to core MAINTAINERS.txt.

Remaining tasks

- Confirm with @lostcarpark their existing activity and interest.
- The Mentoring Working Group (MWG) to gather feedback from current mentoring coordinators.
- Move this issue into Drupal Core so a merge request can be created.