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

test/html/selectors_descendant.wren

@@ -0,0 +1,17 @@
+// retoor <retoor@molodetz.nl>
+
+import "html" for Document
+
+var doc = Document.parse("<div><section><p>Deep</p></section></div>")
+
+System.print(doc.querySelector("div p") != null) // expect: true
+System.print(doc.querySelector("div section") != null) // expect: true
+System.print(doc.querySelector("section p") != null) // expect: true
+System.print(doc.querySelector("div section p") != null) // expect: true
+
+System.print(doc.querySelector("div span") == null) // expect: true
+
+var multi = Document.parse("<div><p>A</p><span><p>B</p></span></div>")
+System.print(multi.querySelectorAll("div p").count) // expect: 2
+System.print(multi.querySelectorAll("span p").count) // expect: 1
+