tpl/tplimpl: Add img loading attribute to figure shortcode (#10927)

docs/content/en/content-management/shortcodes.md

@@ -117,6 +117,9 @@ height
 width
 : `width` attribute of the image.
 
+loading
+: `loading` attribute of the image.
+
 attr
 : Image attribution text. Markdown within the value of `attr` will be rendered.
 

tpl/tplimpl/embedded/templates/shortcodes/figure.html

@@ -8,6 +8,7 @@
          {{- end -}}
          {{- with .Get "width" }} width="{{ . }}"{{ end -}}
          {{- with .Get "height" }} height="{{ . }}"{{ end -}}
+         {{- with .Get "loading" }} loading="{{ . }}"{{ end -}}
     /><!-- Closing img tag -->
     {{- if .Get "link" }}</a>{{ end -}}
     {{- if or (or (.Get "title") (.Get "caption")) (.Get "attr") -}}