Install RSpecMatcherProxies via Object#extend as well

[amatsuda]

Here's a fix for the Capybara vs. Cucumber vs. Aruba's all method conflict problem described in #2761

Cucumber's implementation is that it creates a world by mixing-in various modules on top of an anonymous module: https://github.com/cucumber/cucumber-ruby/blob/v9.2.0/lib/cucumber/glue/proto_world.rb#L176

and Capybara::DSL is mixed-in in this process via Object#extend. Not Module#include nor Module#prepend.

Thus, our DSLRSpecProxyInstaller module has to stand by for self.extended hook in addition to currently implemented ones.

I confirmed that this patch fixes @PragTob's reproduction case in https://github.com/PragTob/all_conflict

fixes #2761