Added XML encoding/decoding (#1067)

* Added XML encoding/decoding

* Minor fixes

* Improve yq doc

* Lint

pkg/yqlib/encoder_json.go

@@ -0,0 +1,64 @@
+package yqlib
+
+import (
+	"encoding/json"
+	"io"
+
+	yaml "gopkg.in/yaml.v3"
+)
+
+type jsonEncoder struct {
+	indentString string
+}
+
+func mapKeysToStrings(node *yaml.Node) {
+
+	if node.Kind == yaml.MappingNode {
+		for index, child := range node.Content {
+			if index%2 == 0 { // its a map key
+				child.Tag = "!!str"
+			}
+		}
+	}
+
+	for _, child := range node.Content {
+		mapKeysToStrings(child)
+	}
+}
+
+func NewJsonEncoder(indent int) Encoder {
+	var indentString = ""
+
+	for index := 0; index < indent; index++ {
+		indentString = indentString + " "
+	}
+
+	return &jsonEncoder{indentString}
+}
+
+func (je *jsonEncoder) CanHandleAliases() bool {
+	return false
+}
+
+func (je *jsonEncoder) PrintDocumentSeparator(writer io.Writer) error {
+	return nil
+}
+
+func (je *jsonEncoder) PrintLeadingContent(writer io.Writer, content string) error {
+	return nil
+}
+
+func (je *jsonEncoder) Encode(writer io.Writer, node *yaml.Node) error {
+	var encoder = json.NewEncoder(writer)
+	encoder.SetEscapeHTML(false) // do not escape html chars e.g. &, <, >
+	encoder.SetIndent("", je.indentString)
+
+	var dataBucket orderedMap
+	// firstly, convert all map keys to strings
+	mapKeysToStrings(node)
+	errorDecoding := node.Decode(&dataBucket)
+	if errorDecoding != nil {
+		return errorDecoding
+	}
+	return encoder.Encode(dataBucket)
+}