Update shell.handlebars font backward

Explanation of Changes
Backward Compatibility:

The font_custom variable is checked to see if it is an array. If not, it is converted to an array using the array helper function.
Handling Multiple Fonts:

The rest of the logic remains the same to handle multiple fonts and generate the necessary @font-face rules and CSS variables.
This approach ensures that the code works correctly whether font_custom is passed as a single string or as a list, maintaining backward compatibility.

Author: amrutadotorg

sqlpage/templates/shell.handlebars

@@ -16,6 +16,14 @@
         {{/if}}
     {{/each}}
 
+    {{!-- Ensure font_custom is an array --}}
+    {{#if font_custom}}
+        {{#if (not (isArray font_custom))}}
+            {{!-- Wrap single font string in an array --}}
+            {{#set "font_custom" (array font_custom)}}
+        {{/if}}
+    {{/if}}
+
     {{#if font_custom}}
         <style>
             {{#each font_custom}}