[markdown] attach indent/outdent keys to toolbar buttons for cross platform support

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/98111



Merged-by: Phil Hughes <me@iamphill.com>
Approved-by: Roy Zwambag <rzwambag@gitlab.com>
Approved-by: Eduardo Sanz García <esanz-garcia@gitlab.com>
Approved-by: Phil Hughes <me@iamphill.com>
Co-authored-by: Brett Walker <bwalker@gitlab.com>