feat: toc external link (#187)

Author: Jakub Jankowski

src/TableOfContents/index.tsx

@@ -140,7 +140,7 @@ function TableOfContentsInner<T extends TableOfContentsItem = TableOfContentsIte
         return (
           <RowComponent
             key={index}
-            item={item}
+            item={isExternalLink(item) ? { ...item, isExternalLink: true } : item}
             index={index}
             isExpanded={isExpanded}
             toggleExpanded={toggleExpandedFunctions[index]}
@@ -346,3 +346,11 @@ function findAncestorIndices(currentDepth: number, precedingContents: TableOfCon
     parentIndex,
   ];
 }
+
+function isExternalLink(item: TableOfContentsItem): boolean {
+  return isTableOfContentsLink(item) && item.to !== void 0 && /^(http|#|mailto)/.test(item.to);
+}
+
+function isTableOfContentsLink(item: TableOfContentsItem): item is ITableOfContentsLink {
+  return 'to' in item;
+}