Prior to this commit, the behat_form_editor integration was hard-coded
to use Atto.
This change allows other editors to be used to set editor values, and
breaks that hard bond. Following this change, any editor is able to
define a behat_editor_[name].php file defining a function which meets
the following signature:
set_editor_value(string $editorid, string $value): void
Each editor is responsible for checking whether its own API is available
and calling any relevant functions to force the editor to set the
content.
Please note: Behat is unable to determine the current editor in use on
the page automatically.
