fix(markup): resolve and proxy srcset links of <source> elements (#39708)

1. use correct links for srcset attribute of picture source
2. wrap the picture element with a link correctly

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/html.go

@@ -355,6 +355,8 @@ func visitNode(ctx *RenderContext, procs []processor, node *html.Node) *html.Nod
 		return visitNodeImg(ctx, node)
 	} else if node.Data == "video" {
 		return visitNodeVideo(ctx, node)
+	} else if node.Data == "source" {
+		return visitNodePictureSource(ctx, node)
 	}
 
 	if node.Data == "a" {
@@ -363,10 +365,11 @@ func visitNode(ctx *RenderContext, procs []processor, node *html.Node) *html.Nod
 		// because the content there is not processable, for example: the content is a commit id or a full URL.
 		procs = emojiProcessors
 	}
+	next := node.NextSibling
 	for n := node.FirstChild; n != nil; {
 		n = visitNode(ctx, procs, n)
 	}
-	return node.NextSibling
+	return next
 }
 
 // createKeyword() renders a highlighted version of an action keyword