feat: enable rich text in figure lightbox captions

By adding `safeHTMLAttr`, similarly to the gallery shortcode.

Enables links to show within the figure lightbox, for example.

Close #1706

layouts/shortcodes/figure.html

@@ -21,7 +21,7 @@
 <figure{{ with .Get "class" }} class="{{.}}"{{ end }} {{ with $caption }}id="figure-{{ anchorize . }}"{{ end }}>
 
 {{ if $lightbox }}
-  <a data-fancybox="{{$group}}" href="{{$image_src}}" {{ with $caption }}data-caption="{{ .|markdownify|emojify }}"{{ end }}>
+  <a data-fancybox="{{$group}}" href="{{$image_src}}" {{ with $caption }}data-caption="{{ .|markdownify|emojify|safeHTMLAttr }}"{{ end }}>
 {{ else if .Get "link"}}
   <a href="{{ .Get "link" }}"{{ with .Get "target" }} target="{{.}}"{{ end }}{{ with .Get "rel" }} rel="{{.}}"{{ end }}>
 {{ end -}}