Install RSpecMatcherProxies via Object#extend as well

Because that's how Cucumber world module mixes-in Capybara::DSL
https://github.com/cucumber/cucumber-ruby/blob/v9.2.0/lib/cucumber/glue/proto_world.rb#L176

fixes teamcapybara#2761

lib/capybara/rspec/matcher_proxies.rb

@@ -52,6 +52,11 @@ def included(base)
         base.include(::Capybara::RSpecMatcherProxies) if defined?(::RSpec::Matchers) && base.include?(::RSpec::Matchers)
         super
       end
+
+      def extended(base)
+        base.extend(::Capybara::RSpecMatcherProxies) if defined?(::RSpec::Matchers) && base.is_a?(::RSpec::Matchers)
+        super
+      end
     end
 
     def self.prepended(base)