Fix strict parsing of find variable with a name expression

lib/liquid/parser.rb

@@ -52,6 +52,11 @@ module Liquid
       when :id
         str = consume
         str << variable_lookups
+      when :open_square
+        str = consume
+        str << expression
+        str << consume(:close_square)
+        str << variable_lookups
       when :string, :number
         consume
       when :open_round

test/integration/variable_test.rb

@@ -95,4 +95,8 @@ class VariableTest < Minitest::Test
   def test_render_symbol
     assert_template_result('bar', '{{ foo }}', 'foo' => :bar)
   end
+
+  def test_dynamic_find_var
+    assert_template_result('bar', '{{ [key] }}', 'key' => 'foo', 'foo' => 'bar')
+  end
 end