LibSyntax+LibGUI+LibJS: Move JS syntax highlighter to LibJS

This is a little bit messy but the basic idea is:

Syntax::Highlighter now has a Syntax::HighlighterClient to talk to the
outside world. It mostly communicates in LibGUI primitives that are
available in headers, so inlineable.

GUI::TextEditor inherits from Syntax::HighlighterClient.

This let us to move GUI::JSSyntaxHighlighter to JS::SyntaxHighlighter
and remove LibGUI's dependency on LibJS.

Userland/Applications/Browser/BrowserConsoleClient.cpp

@@ -28,13 +28,10 @@
 #include "ConsoleWidget.h"
 #include <AK/StringBuilder.h>
 #include <LibGUI/BoxLayout.h>
-#include <LibGUI/JSSyntaxHighlighter.h>
 #include <LibGUI/TextBox.h>
 #include <LibWeb/DOM/DocumentType.h>
-#include <LibWeb/DOM/ElementFactory.h>
 #include <LibWeb/DOM/Text.h>
 #include <LibWeb/DOMTreeModel.h>
-#include <LibWeb/HTML/HTMLBodyElement.h>
 
 namespace Browser {