markup/goldmark: Make auto IDs GitHub compatible

You can turn off this behaviour:

```toml
[markup]
  [markup.goldmark]
    [markup.goldmark.parser]
      autoHeadingIDAsciiOnly = true
```
Note that the `anchorize` now adapts its behaviour depending on the default Markdown handler.

Fixes #6616

markup/converter/converter.go

@@ -87,6 +87,11 @@ type TableOfContentsProvider interface {
 	TableOfContents() tableofcontents.Root
 }
 
+// AnchorNameSanitizer tells how a converter sanitizes anchor names.
+type AnchorNameSanitizer interface {
+	SanitizeAnchorName(s string) string
+}
+
 // Bytes holds a byte slice and implements the Result interface.
 type Bytes []byte