encoding.xml: make tag name and attribute parsing more robust and cleaner (#19828)

vlib/encoding/xml/test/local/18_single_letter_tag/shared.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 <sst count="5" uniqueCount="5">
   <si>
-    <t>Item 1</t>
+    <t a="1">Item 1</t>
   </si>
   <si>
     <t>Item 2</t>

vlib/encoding/xml/test/local/18_single_letter_tag/shared_test.v

@@ -19,6 +19,9 @@ fn test_valid_parsing() {
 					children: [
 						xml.XMLNode{
 							name: 't'
+							attributes: {
+								'a': '1'
+							}
 							children: ['Item 1']
 						},
 					]