Add option legacy file which will load removed constants

test/integration/legacy_test.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+require 'liquid/legacy'
+
+class FiltersTest < Minitest::Test
+  include Liquid
+
+  def test_constants
+    assert_equal Liquid::FilterSeparator, Liquid::FILTER_SEPARATOR
+    assert_equal Liquid::BlockBody::ContentOfVariable, Liquid::BlockBody::CONTENT_OF_VARIABLE
+  end
+end