Update resource limit handling

This commit aligns snippet tag's resource
limit handling with assign tag's `assign_score_of`
method where the snippet variable acts as a pointer
to a block body - increasing assign score by one

test/integration/tags/snippet_test.rb

@@ -1081,7 +1081,7 @@ class SnippetTest < Minitest::Test
     def test_increment_assign_score_by_bytes_not_characters
       t = Template.parse("{% snippet foo %}すごい{% endsnippet %}")
       t.render!
-      assert_equal(9, t.resource_limits.assign_score)
+      assert_equal(1, t.resource_limits.assign_score)
     end
   end
 end