LibWeb: Stop parsing after document.write at the insertion point

If a call to `document.write` inserts an incomplete HTML tag, e.g.:

    document.write("<p");

we would previously continue parsing the document until we reached a
closing angle bracket. However, the spec states we should stop once we
reach the new insertion point.

Userland/Libraries/LibWeb/HTML/Parser/HTMLTokenizer.h

@@ -111,7 +111,11 @@ public:
 #undef __ENUMERATE_TOKENIZER_STATE
     };
 
-    Optional<HTMLToken> next_token();
+    enum class StopAtInsertionPoint {
+        No,
+        Yes,
+    };
+    Optional<HTMLToken> next_token(StopAtInsertionPoint = StopAtInsertionPoint::No);
 
     void set_parser(Badge<HTMLParser>, HTMLParser& parser) { m_parser = &parser; }