Add social icon for substack (adityatelange#1129)

Author: shivan-s

layouts/partials/svg.html

@@ -584,6 +584,12 @@
     <path
         d="M15.387 17.944l-2.089-4.116h-3.065L15.387 24l5.15-10.172h-3.066m-7.008-5.599l2.836 5.598h4.172L10.463 0l-7 13.828h4.169"/>
 </svg>
+</svg>
+{{- else if (eq $icon_name "substack") -}}
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="currentColor" stroke-width="2">
+    <path d="M22.539 8.242H1.46V5.406h21.08v2.836zM1.46 10.812V24L12 18.11 22.54 24V10.812H1.46zM22.54 0H1.46v2.836h21.08V0z"/>
+</svg>
+
 {{- else if (eq $icon_name "telegram") -}}
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"
     stroke-linecap="round" stroke-linejoin="round">