Feature/xmltree additions (#20988)

* [change] add/insert/delete family of xmltree expanded with several variations. Added replace methods family

* [change] Lifted child limitations on insert methods (consulted with @araq)

* [tests] add/insert/replace/delete of xmltree XmlNodes tests added

tests/xml/ttree_add.nim

@@ -0,0 +1,51 @@
+discard """
+  output: '''
+<body>
+  <div>Some text in body</div>
+  <div>Some more text in body </div>
+</body>
+<xml>
+  <head>
+    <div>Some text</div>
+    <div>Some more text </div>
+  </head>
+  <body>
+    <div>Some text in body</div>
+    <div>Some more text in body </div>
+  </body>
+</xml>
+'''
+"""
+
+# Test xmltree add/insert/delete/replace operations
+import xmlparser
+import xmltree
+var baseDocHead = """
+<xml>
+  <head>
+    <div>Some text</div>
+    <div>Some more text </div>
+  </head>
+</xml>
+"""
+var baseDocHeadTree = parseXml(baseDocHead)
+var baseDocBody = """
+<body>
+  <div>Some text in body</div>
+  <div>Some more text in body </div>
+</body>
+"""
+var baseDocBodyTree = parseXml(baseDocBody)
+
+proc test_add() =
+  var testDoc = baseDocHeadTree
+  var newBody = newElement("body")
+  for item in baseDocBodyTree.items():
+    newBody.add(item)
+  
+  echo $newBody
+  
+  testDoc.add(newBody)
+  echo $testDoc
+
+test_add()