LibWeb: Remove most uses of the old HTML parser

The only remaining client of the old parser is the fragment parser used
by the Element.innerHTML setter. We'll need to implement a bit more
stuff in the new parser before we can switch that over.

Libraries/LibWeb/Parser/HTMLDocumentParser.h

@@ -59,6 +59,8 @@
 
 namespace Web {
 
+RefPtr<Document> parse_html_document(const StringView&, const URL&, const String& encoding);
+
 class HTMLDocumentParser {
 public:
     HTMLDocumentParser(const StringView& input, const String& encoding);