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

markup/markup.go

@@ -16,6 +16,12 @@ package markup
 import (
 	"strings"
 
+	"github.com/gohugoio/hugo/markup/highlight"
+
+	"github.com/gohugoio/hugo/markup/markup_config"
+
+	"github.com/gohugoio/hugo/markup/goldmark"
+
 	"github.com/gohugoio/hugo/markup/org"
 
 	"github.com/gohugoio/hugo/markup/asciidoc"
@@ -29,39 +35,71 @@ import (
 func NewConverterProvider(cfg converter.ProviderConfig) (ConverterProvider, error) {
 	converters := make(map[string]converter.Provider)
 
-	add := func(p converter.NewProvider, aliases ...string) error {
+	markupConfig, err := markup_config.Decode(cfg.Cfg)
+	if err != nil {
+		return nil, err
+	}
+
+	if cfg.Highlight == nil {
+		h := highlight.New(markupConfig.Highlight)
+		cfg.Highlight = func(code, lang, optsStr string) (string, error) {
+			return h.Highlight(code, lang, optsStr)
+		}
+	}
+
+	cfg.MarkupConfig = markupConfig
+
+	add := func(p converter.ProviderProvider, aliases ...string) error {
 		c, err := p.New(cfg)
 		if err != nil {
 			return err
 		}
+
+		name := c.Name()
+
+		aliases = append(aliases, name)
+
+		if strings.EqualFold(name, cfg.MarkupConfig.DefaultMarkdownHandler) {
+			aliases = append(aliases, "markdown")
+		}
+
 		addConverter(converters, c, aliases...)
 		return nil
 	}
 
-	if err := add(blackfriday.Provider, "md", "markdown", "blackfriday"); err != nil {
+	if err := add(goldmark.Provider); err != nil {
 		return nil, err
 	}
-	if err := add(mmark.Provider, "mmark"); err != nil {
+	if err := add(blackfriday.Provider); err != nil {
 		return nil, err
 	}
-	if err := add(asciidoc.Provider, "asciidoc"); err != nil {
+	if err := add(mmark.Provider); err != nil {
 		return nil, err
 	}
-	if err := add(rst.Provider, "rst"); err != nil {
+	if err := add(asciidoc.Provider, "ad", "adoc"); err != nil {
 		return nil, err
 	}
-	if err := add(pandoc.Provider, "pandoc"); err != nil {
+	if err := add(rst.Provider); err != nil {
 		return nil, err
 	}
-	if err := add(org.Provider, "org"); err != nil {
+	if err := add(pandoc.Provider, "pdc"); err != nil {
+		return nil, err
+	}
+	if err := add(org.Provider); err != nil {
 		return nil, err
 	}
 
-	return &converterRegistry{converters: converters}, nil
+	return &converterRegistry{
+		config:     cfg,
+		converters: converters,
+	}, nil
 }
 
 type ConverterProvider interface {
 	Get(name string) converter.Provider
+	//Default() converter.Provider
+	GetMarkupConfig() markup_config.Config
+	Highlight(code, lang, optsStr string) (string, error)
 }
 
 type converterRegistry struct {
@@ -70,12 +108,22 @@ type converterRegistry struct {
 	// may be registered multiple times.
 	// All names are lower case.
 	converters map[string]converter.Provider
+
+	config converter.ProviderConfig
 }
 
 func (r *converterRegistry) Get(name string) converter.Provider {
 	return r.converters[strings.ToLower(name)]
 }
 
+func (r *converterRegistry) Highlight(code, lang, optsStr string) (string, error) {
+	return r.config.Highlight(code, lang, optsStr)
+}
+
+func (r *converterRegistry) GetMarkupConfig() markup_config.Config {
+	return r.config.MarkupConfig
+}
+
 func addConverter(m map[string]converter.Provider, c converter.Provider, aliases ...string) {
 	for _, alias := range aliases {
 		m[alias] = c