Package rename transition

So in Struts 7.0, the com.opensymphony.xwork2 package is being renamed
and merged with org.apache.struts2.

For many applications this will be a straightforward find and replace
when upgrading to 7.0. However, I was doing some investigation into
whether we can provide a small deprecation/transitionary period where
both packages are supported (e.g. Struts 6.7). This would be very
helpful for migrating large multi-module codebases.

After some work [1], I was able to get key APIs working in both
packages. With these changes, most Action, Result and Interceptor
classes can be migrated over to the new org.apache.struts2 namespace.
I've also created compatible org.apache.struts2 counterparts for
com.opensymphony.xwork2.ActionContext as well as a bunch of Action
marker interfaces.

I've taken care to ensure these changes don't impact backwards
compatibility and so migrating to the new package namespace as part of
Struts 6.x is entirely optional.

I'm not sure if there is another Struts 6.x release planned but I
think there's some value in providing such a transitional version,
even if it's only for a subset of APIs. Thoughts?

[1] https://github.com/apache/struts/pulls/kusalk