LibWeb: Implement first draft of CSS syntax highlighting :^)

This works at the Token level, which is quick and easy but has
drawbacks: We don't know when something is a property name or a value,
or if something is part of a selector. But, this works for now.

Userland/Libraries/LibSyntax/Highlighter.h

@@ -16,6 +16,7 @@ namespace Syntax {
 
 enum class Language {
     Cpp,
+    CSS,
     GML,
     HTML,
     INI,