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/Libraries/LibGUI/Forward.h

@@ -82,6 +82,7 @@ class TableView;
 class TextBox;
 class TextDocument;
 class TextDocumentLine;
+struct TextDocumentSpan;
 class TextDocumentUndoCommand;
 class TextEditor;
 class ThemeChangeEvent;