Issue #1939102 by Cottser, joelpittet, gnuget, mikl: Convert theme_indentation() to Twig.

core/modules/system/templates/indentation.html.twig

+{#
+/**
+ * @file
+ * Default theme implementation for a set of indentation divs.
+ *
+ * These <div> tags are used for drag and drop tables.
+ *
+ * Available variables:
+ * - size: Optional. The number of indentations to create.
+ *
+ * @ingroup themeable
+ */
+#}
+<!--
+THIS FILE IS NOT USED AND IS HERE AS A STARTING POINT FOR CUSTOMIZATION ONLY.
+See https://api.drupal.org/api/function/theme_indentation/8 for details.
+After copying this file to your theme's folder and customizing it, remove this
+HTML comment.
+-->
+{% for i in 1..size if size > 0 %}<div class="indentation">&nbsp;</div>{% endfor %}