Fix visitListItem not being called by MarkupVisitor protocol dispatch

The existing `visitListItem(_:orderedIndex:)` method has a second
defaulted parameter, which means it does not satisfy the
`MarkupVisitor` protocol requirement `visitListItem(_:)`. Swift
resolves the protocol dispatch to the default extension implementation
(which calls `defaultVisit`) instead of this type's method, so list
item prefixes (bullets, numbers) are never inserted and newlines
between items are lost.

Add a one-parameter overload that matches the protocol signature and
delegates to the two-parameter version.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: SkylerSea

Sources/MarkdownToAttributedString/AttributedStringVisitor.swift

@@ -306,6 +306,13 @@ struct AttributedStringVisitor: MarkupVisitor {
     }
 
 
+    // Satisfies the single-parameter MarkupVisitor protocol requirement so that
+    // protocol dispatch (e.g. markup.accept(&self)) resolves to this type's
+    // implementation rather than the default provided by the protocol extension.
+    mutating func visitListItem(_ listItem: ListItem) {
+        visitListItem(listItem, orderedIndex: nil)
+    }
+
     // orderedIndex is non-nil when part of an ordered list
     mutating func visitListItem(_ listItem: ListItem, orderedIndex: Int? = nil) {
         guard optionsSupportEl(.listItem) else {