feat: use the native crystal highlighter

The chroma highlighter for crystal is not great,
because the pygments one special cases things like
heredocs and that got lost in translation.

Since the crystal compiler comes with a highlighter
why not use it?

src/styles.cr

@@ -84,27 +84,6 @@ module Tartrazine
 
     property styles = {} of String => Style
 
-    # Get the style for a token.
-    def style(token)
-      styles[token] = Style.new unless styles.has_key?(token)
-      s = styles[token]
-
-      # We already got the data from the style hierarchy
-      return s if s.complete?
-
-      # Form the hierarchy of parent styles
-      parents = style_parents(token)
-
-      s = parents.map do |parent|
-        styles[parent]
-      end.reduce(s) do |acc, style|
-        acc + style
-      end
-      s.complete = true
-      styles[token] = s
-      s
-    end
-
     def style_parents(token)
       parents = ["Background"]
       parts = token.underscore.split("_").map(&.capitalize)