[FIX] web_editor: carriage return in source mode

Enter keypress browser event were removed in 3c372d1d.

It was reintroduced in text fields with d2f024d2 and this PR do the
same and reintroduce it for source mode of FieldHtml (`html` widget).

opw-2302605
closes #55066

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

addons/web_editor/static/src/js/backend/field_html.js

@@ -450,6 +450,19 @@ var FieldHtml = basic_fields.DebouncedField.extend(TranslatableFieldMixin, {
             $(this).attr('id', 'checklist-id-' + (++max));
         });
     },
+    /**
+     * Allows Enter keypress in a textarea (source mode)
+     *
+     * @private
+     * @param {OdooEvent} ev
+     */
+    _onKeydown: function (ev) {
+        if (ev.which === $.ui.keyCode.ENTER && $(ev.target).is('textarea')) {
+            ev.stopPropagation();
+            return;
+        }
+        this._super.apply(this, arguments);
+    },
     /**
      * Method called when wysiwyg triggers a change.
      *