MDL-75887 behat: Add ability to set value regardless of the editor

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.

lib/behat/classes/settable_editor.php

@@ -0,0 +1,37 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Editor interface for setting editor values.
+ *
+ * @package    behat
+ * @category   test
+ * @copyright  2022 Andrew Lyons <[email protected]>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace core_behat;
+
+interface settable_editor {
+
+    /**
+     * Set the editor value.
+     *
+     * @param string $editorid The id of the editor within the page
+     * @param string $value The intended content of the editor
+     */
+    public function set_editor_value(string $editorid, string $value): void;
+}