Use VariableSignature as Syntax for Capture tag to allow hyphens in variable names

test/integration/assign_test.rb

@@ -3,6 +3,16 @@ require 'test_helper'
 class AssignTest < Minitest::Test
   include Liquid
 
+  def test_assign_with_hyphen_in_variable_name
+    template_source = <<-END_TEMPLATE
+    {% assign this-thing = 'Print this-thing' %}
+    {{ this-thing }}
+    END_TEMPLATE
+    template = Template.parse(template_source)
+    rendered = template.render!
+    assert_equal "Print this-thing", rendered.strip
+  end
+
   def test_assigned_variable
     assert_template_result('.foo.',
                            '{% assign foo = values %}.{{ foo[0] }}.',