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/rst/convert.go

@@ -25,20 +25,18 @@ import (
 )
 
 // Provider is the package entry point.
-var Provider converter.NewProvider = provider{}
+var Provider converter.ProviderProvider = provider{}
 
 type provider struct {
 }
 
 func (p provider) New(cfg converter.ProviderConfig) (converter.Provider, error) {
-	var n converter.NewConverter = func(ctx converter.DocumentContext) (converter.Converter, error) {
+	return converter.NewProvider("rst", func(ctx converter.DocumentContext) (converter.Converter, error) {
 		return &rstConverter{
 			ctx: ctx,
 			cfg: cfg,
 		}, nil
-	}
-	return n, nil
-
+	}), nil
 }
 
 type rstConverter struct {