tpl/transform: Add template func for TOML/JSON/YAML docs examples conversion

Usage:

```html
{{ "title = \"Hello World\"" | transform.Remarshal "json" | safeHTML }}
```

Fixes #4389

tpl/transform/remarshal.go

@@ -0,0 +1,98 @@
+package transform
+
+import (
+	"bytes"
+	"errors"
+	"strings"
+
+	"github.com/gohugoio/hugo/parser"
+	"github.com/spf13/cast"
+)
+
+// Remarshal is used in the Hugo documentation to convert configuration
+// examples from YAML to JSON, TOML (and possibly the other way around).
+// The is primarily a helper for the Hugo docs site.
+// It is not a general purpose YAML to TOML converter etc., and may
+// change without notice if it serves a purpose in the docs.
+// Format is one of json, yaml or toml.
+func (ns *Namespace) Remarshal(format string, data interface{}) (string, error) {
+	from, err := cast.ToStringE(data)
+	if err != nil {
+		return "", err
+	}
+
+	from = strings.TrimSpace(from)
+	format = strings.TrimSpace(strings.ToLower(format))
+
+	if from == "" {
+		return "", nil
+	}
+
+	mark, err := toFormatMark(format)
+	if err != nil {
+		return "", err
+	}
+
+	fromFormat, err := detectFormat(from)
+	if err != nil {
+		return "", err
+	}
+
+	var metaHandler func(d []byte) (map[string]interface{}, error)
+
+	switch fromFormat {
+	case "yaml":
+		metaHandler = parser.HandleYAMLMetaData
+	case "toml":
+		metaHandler = parser.HandleTOMLMetaData
+	case "json":
+		metaHandler = parser.HandleJSONMetaData
+	}
+
+	meta, err := metaHandler([]byte(from))
+	if err != nil {
+		return "", err
+	}
+
+	var result bytes.Buffer
+	if err := parser.InterfaceToConfig(meta, mark, &result); err != nil {
+		return "", err
+	}
+
+	return result.String(), nil
+}
+
+func toFormatMark(format string) (rune, error) {
+	// TODO(bep) the parser package needs a cleaning.
+	switch format {
+	case "yaml":
+		return rune(parser.YAMLLead[0]), nil
+	case "toml":
+		return rune(parser.TOMLLead[0]), nil
+	case "json":
+		return rune(parser.JSONLead[0]), nil
+	}
+
+	return 0, errors.New("failed to detect target data serialization format")
+}
+
+func detectFormat(data string) (string, error) {
+	jsonIdx := strings.Index(data, "{")
+	yamlIdx := strings.Index(data, ":")
+	tomlIdx := strings.Index(data, "=")
+
+	if jsonIdx != -1 && (yamlIdx == -1 || jsonIdx < yamlIdx) && (tomlIdx == -1 || jsonIdx < tomlIdx) {
+		return "json", nil
+	}
+
+	if yamlIdx != -1 && (tomlIdx == -1 || yamlIdx < tomlIdx) {
+		return "yaml", nil
+	}
+
+	if tomlIdx != -1 {
+		return "toml", nil
+	}
+
+	return "", errors.New("failed to detect data serialization format")
+
+}