Fix goldmark toc rendering

Previously gordmark-based TOC renderes only `KindText` and `KindString`

This commit expands target node with Goldmark's renderer

I am not sure of what are expected results as TOC contents in some (rare) cases
but Blackfriday's behaviours are fundamentally respected.

For example,
- image `[image text](link)` is rendered as `<img>` tag
- GFM AutoLink `gohugo.io` is rendered as text

* Render AutoLink as <a> tag as Blackfriday does

Fixes #6736
Fixes #6809

hugolib/shortcode_test.go

@@ -1136,7 +1136,7 @@ TOC: {{ .TableOfContents }}
 					"NEW INLINE:  W1: [1 2 3 4 5]",
 					"INLINE IN INNER: Inner: W2: [1 2 3 4]",
 					"REUSED INLINE IN INNER: Inner: W1: [1 2 3]",
-					`<li><a href="#markdown-delimiter-hugo-rocks">MARKDOWN DELIMITER: Hugo Rocks!</a></li>`,
+					`<li><a href="#markdown-delimiter-hugo-rocks">MARKDOWN DELIMITER: <strong>Hugo Rocks!</strong></a></li>`,
 				}
 
 				if enableInlineShortcodes {