Fix heading auto-link hijacking nested link labels (#668)

src/Markdig/Extensions/AutoIdentifiers/HeadingLinkReferenceDefinition.cs

@@ -24,4 +24,11 @@ public class HeadingLinkReferenceDefinition : LinkReferenceDefinition
     /// Gets or sets the heading related to this link reference definition.
     /// </summary>
     public HeadingBlock Heading { get; set; }
+
+    /// <inheritdoc />
+    /// <remarks>
+    /// Created implicitly for every heading, so it must not resolve inside another
+    /// still-open link bracket, e.g. <c>[Some text [Heading]](url)</c>.
+    /// </remarks>
+    internal override bool AllowResolutionInsideOpenLink => false;
 }