LibWeb: Factor out the "stack of open elements" into its own class

This will allow us to write more expressive parsing code. :^)

Libraries/LibWeb/Parser/StackOfOpenElements.cpp

@@ -0,0 +1,23 @@
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/Parser/StackOfOpenElements.h>
+
+namespace Web {
+
+StackOfOpenElements::~StackOfOpenElements()
+{
+}
+
+bool StackOfOpenElements::has_in_scope(const FlyString& tag_name) const
+{
+    static Vector<FlyString> list { "applet", "caption", "html", "table", "td", "th", "marquee", "object", "template" };
+    for (ssize_t i = m_elements.size() - 1; i >= 0; --i) {
+        auto& node = m_elements.at(i);
+        if (node.tag_name() == tag_name)
+            return true;
+        if (list.contains_slow(node.tag_name()))
+            return false;
+    }
+    ASSERT_NOT_REACHED();
+}
+
+}