Support pretty printing of XML types

[HertzDevil]

Overrides #pretty_print for XML::Node, XML::NodeSet, XML::Attributes, and XML::Namespace, consistent with their #inspect overrides.

Example:

require "xml"

xml = XML.parse(<<-XML)
  <?xml version="1.0" encoding="UTF-8"?>
  <crystal:foo x="1" y="2" xmlns:crystal="https://crystal-lang.org/">
    <bar/>lorem ipsum<baz/>
  </foo>
  XML

pp xml
# #<XML::Document:0x1b9193c2d80
#  children=
#   [#<XML::Element:0x1b9193cdf00
#     name="foo"
#     attributes=
#      [#<XML::Attribute:0x1b9193b3e00 name="x" content="1">,
#       #<XML::Attribute:0x1b9193b3d90 name="y" content="2">]
#     children=
#      [#<XML::Text:0x1b9193cdd80 "\n" + "  ">,
#       #<XML::Element:0x1b9193cdd00 name="bar">,
#       #<XML::Text:0x1b9193cdc80 "lorem ipsum">,
#       #<XML::Element:0x1b9193cdc00 name="baz">,
#       #<XML::Text:0x1b9193cdb80 "\n">]>]>

pp xml.root.not_nil!.namespace
# #<XML::Namespace:0x1b9193b1b00
#  prefix="crystal"
#  href="https://crystal-lang.org/">