Expose MoveToNamespace action in code fixes

[SamPruden]

Background and Motivation

I would like to write an analyzer and codefix to enforce a specific namespace/directory relationship that's a little different from the standard, motivated by the Unity game engine. I would like the codefix to be able to do the equivalent of MoveToNamespace when changing namespaces, but as far as I can tell that functionality isn't exposed.

I tried looking at the MatchFolderAndNamespace codefix for tips, but it looks like that moves the file to match the namespace, which is the opposite of what I want.

It might be possible to chase symbol references throughout the solution and to manually patch them to a changed namespace, but this seems very brittle to problems such as ambiguities introduced by adding new usings to other files. Presumably the infrastructure for doing this well already exists and could be exposed.

Proposed API

I would expect an API similar to Renamer for refactoring namespaces.