Update nav-for-headings.html

I prompted in the existing conversation:

> Make sure to populate existingIds with the IDs on the page to start

Author: simonw

nav-for-headings.html

@@ -109,7 +109,8 @@
   const url = document.getElementById('urlInput').value
   const parser = new DOMParser()
   const doc = parser.parseFromString(html, 'text/html')
-  const existingIds = new Set()
+  // Collect all existing IDs on the page first
+  const existingIds = new Set([...doc.querySelectorAll('[id]')].map(el => el.id))
   const headers = doc.querySelectorAll('h1, h2, h3, h4, h5, h6')
   const headerLinks = []
 
@@ -125,4 +126,4 @@
 }
 </script>
 </body>
-</html>
+</html>