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

markup/goldmark/toc.go

@@ -21,6 +21,7 @@ import (
 	"github.com/yuin/goldmark"
 	"github.com/yuin/goldmark/ast"
 	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/renderer"
 	"github.com/yuin/goldmark/text"
 	"github.com/yuin/goldmark/util"
 )
@@ -31,6 +32,7 @@ var (
 )
 
 type tocTransformer struct {
+	r renderer.Renderer
 }
 
 func (t *tocTransformer) Transform(n *ast.Document, reader text.Reader, pc parser.Context) {
@@ -79,8 +81,26 @@ func (t *tocTransformer) Transform(n *ast.Document, reader text.Reader, pc parse
 			if found {
 				header.ID = string(id.([]byte))
 			}
-		case ast.KindText, ast.KindString:
-			headingText.Write(n.Text(reader.Source()))
+		case
+			ast.KindCodeSpan,
+			ast.KindLink,
+			ast.KindImage,
+			ast.KindEmphasis:
+			err := t.r.Render(&headingText, reader.Source(), n)
+			if err != nil {
+				return s, err
+			}
+
+			return ast.WalkSkipChildren, nil
+		case
+			ast.KindAutoLink,
+			ast.KindRawHTML,
+			ast.KindText,
+			ast.KindString:
+			err := t.r.Render(&headingText, reader.Source(), n)
+			if err != nil {
+				return s, err
+			}
 		}
 
 		return s, nil
@@ -90,12 +110,19 @@ func (t *tocTransformer) Transform(n *ast.Document, reader text.Reader, pc parse
 }
 
 type tocExtension struct {
+	options []renderer.Option
 }
 
-func newTocExtension() goldmark.Extender {
-	return &tocExtension{}
+func newTocExtension(options []renderer.Option) goldmark.Extender {
+	return &tocExtension{
+		options: options,
+	}
 }
 
 func (e *tocExtension) Extend(m goldmark.Markdown) {
-	m.Parser().AddOptions(parser.WithASTTransformers(util.Prioritized(&tocTransformer{}, 10)))
+	r := goldmark.DefaultRenderer()
+	r.AddOptions(e.options...)
+	m.Parser().AddOptions(parser.WithASTTransformers(util.Prioritized(&tocTransformer{
+		r: r,
+	}, 10)))
 }