LibWeb: Fix a few const-ness issues

Author: https://github.com/mattco98
Commit: c0b2fa74ac
Pull-request: https://github.com/SerenityOS/serenity/pull/17620
Issue: https://github.com/SerenityOS/serenity/issues/16988
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibWeb/FileAPI/FileList.h

@@ -24,6 +24,12 @@ public:
     // https://w3c.github.io/FileAPI/#dfn-length
     unsigned long length() const { return m_files.size(); }
 
+    // https://w3c.github.io/FileAPI/#dfn-item
+    File* item(size_t index)
+    {
+        return index < m_files.size() ? m_files[index].ptr() : nullptr;
+    }
+
     // https://w3c.github.io/FileAPI/#dfn-item
     File const* item(size_t index) const
     {