Support refactors that require user input/options

One common request of Dart is a refactor for "move to file" (either moving to an existing file, or a new one). Right now I don't think there's a great way to implement this as far as I can see there's no way to ask the user to pick a file (existing or not).

Slightly related, for Extract Method we would like to ask for a name (there's a related issue at https://github.com/microsoft/language-server-protocol/issues/764 suggesting instead allowing us to trigger a rename at the end, although asking for the name up-front would also work).

There are other refactors that Dart supports in other IDEs that have various other inputs/options (for example "inline method" lets you decide whether to keep or remove the method, and when extracting a method there's an option for whether to replace any other instances of the same statements in the file with a call to the new method).

I'm not sure of the best way to do this, but I thought it was worth starting a discussion (I couldn't find an existing one), but some possible ideas:

• Metadata on code-actions that allow asking for input
• Support for command variables in args of commands on code actions (and well-define input commands)
• Server-to-client commands that allow taking input (similar to window/showMessageRequest)

Start with this issue's discussion and the related Language Server Protocol issue 764. Compare the proposed approaches for collecting refactor inputs, including code-action metadata, command variables, and server-to-client commands. Done would require an agreed protocol design for user-supplied options, rather than a narrowly scoped edit.