Change Node.child return type to immutable.Seq on 2.13+

[lrytz]

Change the type of Node.child and Node.nonEmptyChildren from collection.Seq to immutable.Seq in 2.13+.

A parent is added to Node which defines the old signatures for child / nonEmptyChildren. This (and the resulting bridge methods) ensures binary compatbility.

[lrytz]

If we go ahead with this PR we'll need a new minor release (2.4.0).

There is more API with return type collection.Seq that probably always returns an immutable.Seq, e.g.

• Metadata.value
• NodeSeq.theSeq
• MetaData.apply / MetaData.get / Node.attribute
• TextBuffer.toText
• BasicTransformer.transform

I'm happy take a look at those as well; I only did child for now because that's what is affecting the 2.13 migration I'm working on.

[lrytz]

For the record, there is an implicit conversion collection.Seq[Node] => NodeSeq and NodeSeq is a subclass of immutable.Seq on 2.13.

So the following works already without this PR

scala> val n: Node = <a><b/></a>
val n: scala.xml.Node = <a><b/></a>

scala> val c: collection.immutable.Seq[Node] = n.child
val c: Seq[scala.xml.Node] = Seq(<b/>)

But this fails:

scala> val c: collection.immutable.Seq[String] = n.child.map(_.toString)
                                                            ^
       error: type mismatch;
        found   : Seq[String] (in scala.collection)
        required: Seq[String] (in scala.collection.immutable)