LibJS: Create JS to HTML markup generator

The new JS::MarkupGenerator class can convert both a JS source string
and a JS Runtime Value into properly formatted HTML using the new
LibWeb System Palette css color values.

It makes more sense for this JS -> HTML process to occur in LibJS
so that it can be used elsewhere, namely Markdown code block syntax
highlighting. It also means the Browser can worry less about LibJS
implementation details.

Libraries/LibJS/Forward.h

@@ -73,6 +73,7 @@ class ScopeNode;
 class Shape;
 class Statement;
 class Symbol;
+class Token;
 class Uint8ClampedArray;
 class Value;
 enum class DeclarationKind;