feat: add complete DOM implementation with Document, Element, and NodeList classes to html module

test/html/selectors_pseudo_lastchild.wren

@@ -0,0 +1,15 @@
+// retoor <retoor@molodetz.nl>
+
+import "html" for Document
+
+var doc = Document.parse("<ul><li>A</li><li>B</li><li>C</li></ul>")
+
+System.print(doc.querySelector("li:last-child") != null) // expect: true
+System.print(doc.querySelector("li:last-child").textContent) // expect: C
+
+var multi = Document.parse("<div><p>First</p><span>Last</span></div>")
+System.print(multi.querySelector("span:last-child") != null) // expect: true
+System.print(multi.querySelector("span:last-child").textContent) // expect: Last
+
+System.print(multi.querySelector("p:last-child") == null) // expect: true
+