Added XML encoding/decoding (#1067)

* Added XML encoding/decoding

* Minor fixes

* Improve yq doc

* Lint

pkg/yqlib/encoder_yaml.go

@@ -0,0 +1,101 @@
+package yqlib
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+
+	yaml "gopkg.in/yaml.v3"
+)
+
+type yamlEncoder struct {
+	indent             int
+	colorise           bool
+	printDocSeparators bool
+	unwrapScalar       bool
+}
+
+func NewYamlEncoder(indent int, colorise bool, printDocSeparators bool, unwrapScalar bool) Encoder {
+	if indent < 0 {
+		indent = 0
+	}
+	return &yamlEncoder{indent, colorise, printDocSeparators, unwrapScalar}
+}
+
+func (ye *yamlEncoder) CanHandleAliases() bool {
+	return true
+}
+
+func (ye *yamlEncoder) PrintDocumentSeparator(writer io.Writer) error {
+	if ye.printDocSeparators {
+		log.Debug("-- writing doc sep")
+		if err := writeString(writer, "---\n"); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (ye *yamlEncoder) PrintLeadingContent(writer io.Writer, content string) error {
+	// log.Debug("headcommentwas [%v]", content)
+	reader := bufio.NewReader(strings.NewReader(content))
+
+	for {
+
+		readline, errReading := reader.ReadString('\n')
+		if errReading != nil && !errors.Is(errReading, io.EOF) {
+			return errReading
+		}
+		if strings.Contains(readline, "$yqDocSeperator$") {
+
+			if err := ye.PrintDocumentSeparator(writer); err != nil {
+				return err
+			}
+
+		} else {
+			if err := writeString(writer, readline); err != nil {
+				return err
+			}
+		}
+
+		if errors.Is(errReading, io.EOF) {
+			if readline != "" {
+				// the last comment we read didn't have a new line, put one in
+				if err := writeString(writer, "\n"); err != nil {
+					return err
+				}
+			}
+			break
+		}
+	}
+
+	return nil
+}
+
+func (ye *yamlEncoder) Encode(writer io.Writer, node *yaml.Node) error {
+
+	if node.Kind == yaml.ScalarNode && ye.unwrapScalar {
+		return writeString(writer, node.Value+"\n")
+	}
+
+	destination := writer
+	tempBuffer := bytes.NewBuffer(nil)
+	if ye.colorise {
+		destination = tempBuffer
+	}
+
+	var encoder = yaml.NewEncoder(destination)
+
+	encoder.SetIndent(ye.indent)
+
+	if err := encoder.Encode(node); err != nil {
+		return err
+	}
+
+	if ye.colorise {
+		return colorizeAndPrint(tempBuffer.Bytes(), writer)
+	}
+	return nil
+}