LibWeb: Add basic support for dynamic markup insertion

This implements basic support for dynamic markup insertion, adding
 * Document::open()
 * Document::write(Vector<String> const&)
 * Document::writeln(Vector<String> const&)
 * Document::close()

The HTMLParser is modified to make it possible to create a
script-created parser which initially only contains a HTMLTokenizer
without any data. Aditionally the HTMLParser::run method gains an
overload which does not modify the Document and does not run
HTMLParser::the_end() so that we can reenter the parser at a later time.
Furthermore all FIXMEs that consern the insertion point are implemented
wich is defined in the HTMLTokenizer. Additionally the following
member-variables of the HTMLParser are now exposed by getter funcions:
 * m_tokenizer
 * m_aborted
 * m_script_nesting_level

The HTMLTokenizer is modified so that it contains an insertion
point which keeps track of where the next input from the Document::write
functions will be inserted. The insertion point is implemented as the
charakter offset into m_decoded_input and a boolean describing if the
insertion point is defined. Functions to update, check and {re}store the
insertion point are also added.
The function HTMLTokenizer::insert_eof is added to tell a script-created
parser that document::close was called and HTMLParser::the_end() should
be called.
Lastly an explicit default constructor is added to HTMLTokenizer to
create a empty HTMLTokenizer into which data can be inserted.

Userland/Libraries/LibWeb/DOM/Document.h

@@ -244,8 +244,11 @@ public:
 
     Window& window() { return *m_window; }
 
-    void write(Vector<String> const& strings);
-    void writeln(Vector<String> const& strings);
+    ExceptionOr<void> write(Vector<String> const& strings);
+    ExceptionOr<void> writeln(Vector<String> const& strings);
+
+    ExceptionOr<Document*> open(String const& = "", String const& = "");
+    ExceptionOr<void> close();
 
     Window* default_view() { return m_window; }
 
@@ -355,6 +358,9 @@ private:
     RefPtr<Core::Timer> m_style_update_timer;
     RefPtr<Core::Timer> m_layout_update_timer;
 
+    OwnPtr<HTML::HTMLParser> m_parser;
+    bool m_active_parser_was_aborted { false };
+
     String m_source;
 
     OwnPtr<JS::Interpreter> m_interpreter;
@@ -385,6 +391,12 @@ private:
 
     u32 m_ignore_destructive_writes_counter { 0 };
 
+    // https://html.spec.whatwg.org/multipage/browsing-the-web.html#unload-counter
+    u32 m_unload_counter { 0 };
+
+    // https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#throw-on-dynamic-markup-insertion-counter
+    u32 m_throw_on_dynamic_markup_insertion_counter { 0 };
+
     // https://html.spec.whatwg.org/multipage/semantics.html#script-blocking-style-sheet-counter
     u32 m_script_blocking_style_sheet_counter { 0 };
 
@@ -403,5 +415,4 @@ private:
 
     bool m_needs_layout { false };
 };
-
 }