Fix indentation in highlight shortcode

This commit adds a new `.InnerDeindent` method to the shortcode context, which is `.Inner` with any
indendation removed. This is then used in the built-in `highlight` shortcode to prevent the extra
whitespace getting hightlighted.

Fixes #4717

content/en/variables/shortcodes.md

@@ -45,4 +45,8 @@ toc: false
 [markdownshortcode]: /content-management/shortcodes/#shortcodes-with-markdown
 [shortcodes]: /templates/shortcode-templates/
 
+.InnerDeindent {{< new-in "0.100.0" >}}
+: Gets the `.Inner` with any indentation removed. This is what's used in the built-in `{{</* highlight */>}}` shortcode.
+
+