vlib: add an encoding.xml module with parser, validation, entity encoding, unit tests (#19708)

vlib/encoding/xml/test/local/06_nested_elements/nested.xml

@@ -0,0 +1,14 @@
+<level1>
+  <level2>
+    <level3>
+      <level4>
+        Deeply nested content.
+      </level4>
+    </level3>
+  </level2>
+  <level2>
+    <level3>
+      Less deeply nested content.
+    </level3>
+  </level2>
+</level1>

vlib/encoding/xml/test/local/06_nested_elements/nested_test.v

@@ -0,0 +1,44 @@
+import os
+import encoding.xml
+
+fn test_valid_parsing() ! {
+	path := os.join_path(os.dir(@FILE), 'nested.xml')
+
+	expected := xml.XMLDocument{
+		root: xml.XMLNode{
+			name: 'level1'
+			children: [
+				xml.XMLNode{
+					name: 'level2'
+					children: [
+						xml.XMLNode{
+							name: 'level3'
+							children: [
+								xml.XMLNode{
+									name: 'level4'
+									children: [
+										'Deeply nested content.',
+									]
+								},
+							]
+						},
+					]
+				},
+				xml.XMLNode{
+					name: 'level2'
+					children: [
+						xml.XMLNode{
+							name: 'level3'
+							children: [
+								'Less deeply nested content.',
+							]
+						},
+					]
+				},
+			]
+		}
+	}
+	actual := xml.XMLDocument.from_file(path)!
+
+	assert expected == actual, 'Parsed XML document should be equal to expected XML document'
+}