fix: Haml compatibility

Author: lkovace18

lib/middleman-syntax/haml_monkey_patch.rb

@@ -1,8 +1,7 @@
-# If Haml is around, define a :code filter that can be used to more conveniently output highlighted code.
-if defined? Haml
+if defined? Haml && Gem::Version.new(Haml::VERSION) >= Gem::Version.new("6.0.0.beta.1")
   module Haml
-    module Filters
-      module Code
+    class Filters
+      class Code < Base
         include Base
 
         def render(code)