Add ParseTreeVisitor to Echo tag

This fixes theme-check#218, wherein variables used in echo tags are not
considered used by the linter. It is because our visitor doesn't see the
:variable_lookup's in the echo tag since the children array is empty.
But this array is empty because it is swallowed by the @variable.

test/unit/parse_tree_visitor_test.rb

@@ -26,6 +26,13 @@ class ParseTreeVisitorTest < Minitest::Test
     )
   end
 
+  def test_echo
+    assert_equal(
+      ["test"],
+      visit(%({% echo test %}))
+    )
+  end
+
   def test_if_condition
     assert_equal(
       ["test"],