Add Goldmark as the new default markdown handler

This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.

If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:

```toml
[markup]
defaultMarkdownHandler="blackfriday"
```

Fixes #5963
Fixes #1778
Fixes #6355

1 files changed, 10 insertions, 4 deletions

diff --git a/hugolib/page__meta.go b/hugolib/page__meta.go
index ca5c7007e..1fc69c218 100644
--- a/hugolib/page__meta.go
+++ b/hugolib/page__meta.go
@@ -565,7 +565,7 @@ func (pm *pageMeta) setMetadata(bucket *pagesMapBucket, p *pageState, frontmatte
 		pm.sitemap = p.s.siteCfg.sitemap
 	}
 
-	pm.markup = helpers.GuessType(pm.markup)
+	pm.markup = p.s.ContentSpec.ResolveMarkup(pm.markup)
 
 	if draft != nil && published != nil {
 		pm.draft = *draft
@@ -596,7 +596,7 @@ func (p *pageMeta) applyDefaultValues() error {
 	if p.markup == "" {
 		if !p.File().IsZero() {
 			// Fall back to file extension
-			p.markup = helpers.GuessType(p.File().Ext())
+			p.markup = p.s.ContentSpec.ResolveMarkup(p.File().Ext())
 		}
 		if p.markup == "" {
 			p.markup = "markdown"
@@ -638,14 +638,20 @@ func (p *pageMeta) applyDefaultValues() error {
 		}
 	}
 
-	if !p.f.IsZero() && p.markup != "html" {
+	if !p.f.IsZero() {
 		var renderingConfigOverrides map[string]interface{}
 		bfParam := getParamToLower(p, "blackfriday")
 		if bfParam != nil {
 			renderingConfigOverrides = maps.ToStringMap(bfParam)
 		}
 
-		cp := p.s.ContentSpec.Converters.Get(p.markup)
+		markup := p.markup
+		if markup == "html" {
+			// Only used for shortcode inner content.
+			markup = "markdown"
+		}
+
+		cp := p.s.ContentSpec.Converters.Get(markup)
 		if cp == nil {
 			return errors.Errorf("no content renderer found for markup %q", p.markup)
 		}