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/converter/converter.go

@@ -16,33 +16,51 @@ package converter
 import (
 	"github.com/gohugoio/hugo/common/loggers"
 	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/markup/markup_config"
+	"github.com/gohugoio/hugo/markup/tableofcontents"
 	"github.com/spf13/afero"
 )
 
 // ProviderConfig configures a new Provider.
 type ProviderConfig struct {
+	MarkupConfig markup_config.Config
+
 	Cfg       config.Provider // Site config
 	ContentFs afero.Fs
 	Logger    *loggers.Logger
 	Highlight func(code, lang, optsStr string) (string, error)
 }
 
-// NewProvider creates converter providers.
-type NewProvider interface {
+// ProviderProvider creates converter providers.
+type ProviderProvider interface {
 	New(cfg ProviderConfig) (Provider, error)
 }
 
 // Provider creates converters.
 type Provider interface {
 	New(ctx DocumentContext) (Converter, error)
+	Name() string
 }
 
-// NewConverter is an adapter that can be used as a ConverterProvider.
-type NewConverter func(ctx DocumentContext) (Converter, error)
+// NewProvider creates a new Provider with the given name.
+func NewProvider(name string, create func(ctx DocumentContext) (Converter, error)) Provider {
+	return newConverter{
+		name:   name,
+		create: create,
+	}
+}
 
-// New creates a new Converter for the given ctx.
-func (n NewConverter) New(ctx DocumentContext) (Converter, error) {
-	return n(ctx)
+type newConverter struct {
+	name   string
+	create func(ctx DocumentContext) (Converter, error)
+}
+
+func (n newConverter) New(ctx DocumentContext) (Converter, error) {
+	return n.create(ctx)
+}
+
+func (n newConverter) Name() string {
+	return n.name
 }
 
 // Converter wraps the Convert method that converts some markup into
@@ -61,6 +79,11 @@ type DocumentInfo interface {
 	AnchorSuffix() string
 }
 
+// TableOfContentsProvider provides the content as a ToC structure.
+type TableOfContentsProvider interface {
+	TableOfContents() tableofcontents.Root
+}
+
 // Bytes holds a byte slice and implements the Result interface.
 type Bytes []byte