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/internal/blackfriday.go

@@ -1,108 +0,0 @@
-// Copyright 2019 The Hugo Authors. All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// Package helpers implements general utility functions that work with
-// and on content.  The helper functions defined here lay down the
-// foundation of how Hugo works with files and filepaths, and perform
-// string operations on content.
-
-package internal
-
-import (
-	"github.com/gohugoio/hugo/common/maps"
-	"github.com/gohugoio/hugo/markup/converter"
-	"github.com/mitchellh/mapstructure"
-	"github.com/pkg/errors"
-)
-
-// BlackFriday holds configuration values for BlackFriday rendering.
-// It is kept here because it's used in several packages.
-type BlackFriday struct {
-	Smartypants           bool
-	SmartypantsQuotesNBSP bool
-	AngledQuotes          bool
-	Fractions             bool
-	HrefTargetBlank       bool
-	NofollowLinks         bool
-	NoreferrerLinks       bool
-	SmartDashes           bool
-	LatexDashes           bool
-	TaskLists             bool
-	PlainIDAnchors        bool
-	Extensions            []string
-	ExtensionsMask        []string
-	SkipHTML              bool
-
-	FootnoteAnchorPrefix       string
-	FootnoteReturnLinkContents string
-}
-
-func UpdateBlackFriday(old *BlackFriday, m map[string]interface{}) (*BlackFriday, error) {
-	// Create a copy so we can modify it.
-	bf := *old
-	if err := mapstructure.Decode(m, &bf); err != nil {
-		return nil, errors.WithMessage(err, "failed to decode rendering config")
-	}
-	return &bf, nil
-}
-
-// NewBlackfriday creates a new Blackfriday filled with site config or some sane defaults.
-func NewBlackfriday(cfg converter.ProviderConfig) (*BlackFriday, error) {
-	var siteConfig map[string]interface{}
-	if cfg.Cfg != nil {
-		siteConfig = cfg.Cfg.GetStringMap("blackfriday")
-	}
-
-	defaultParam := map[string]interface{}{
-		"smartypants":           true,
-		"angledQuotes":          false,
-		"smartypantsQuotesNBSP": false,
-		"fractions":             true,
-		"hrefTargetBlank":       false,
-		"nofollowLinks":         false,
-		"noreferrerLinks":       false,
-		"smartDashes":           true,
-		"latexDashes":           true,
-		"plainIDAnchors":        true,
-		"taskLists":             true,
-		"skipHTML":              false,
-	}
-
-	maps.ToLower(defaultParam)
-
-	config := make(map[string]interface{})
-
-	for k, v := range defaultParam {
-		config[k] = v
-	}
-
-	for k, v := range siteConfig {
-		config[k] = v
-	}
-
-	combinedConfig := &BlackFriday{}
-	if err := mapstructure.Decode(config, combinedConfig); err != nil {
-		return nil, errors.Errorf("failed to decode Blackfriday config: %s", err)
-	}
-
-	// TODO(bep) update/consolidate docs
-	if combinedConfig.FootnoteAnchorPrefix == "" {
-		combinedConfig.FootnoteAnchorPrefix = cfg.Cfg.GetString("footnoteAnchorPrefix")
-	}
-
-	if combinedConfig.FootnoteReturnLinkContents == "" {
-		combinedConfig.FootnoteReturnLinkContents = cfg.Cfg.GetString("footnoteReturnLinkContents")
-	}
-
-	return combinedConfig, nil
-}