Move example-specific DSL to its own module

src/spectator/dsl.cr

@@ -168,6 +168,7 @@ module Spectator
       {% given_vars = ::Spectator::ContextDefinitions::ALL[parent_module.id][:given] %}
       {% var_names = given_vars.map { |v| v[:name] } %}
       class {{class_name.id}} < ::Spectator::Example
+        include ExampleDSL
         include Locals
 
         {% if given_vars.empty? %}

src/spectator/example.cr

@@ -7,10 +7,6 @@ module Spectator
     def initialize(@context)
     end
 
-    macro is_expected
-      expect(subject)
-    end
-
     abstract def run : Nil
     abstract def description : String
   end

src/spectator/example_dsl.cr

@@ -0,0 +1,7 @@
+module Spectator
+  module ExampleDSL
+    macro is_expected
+      expect(subject)
+    end
+  end
+end