Provide analyzer + code fix for turning sync command to async and viceversa

[kzu]

The new analyer detects the need for changes in the command definition itself, allowing the command handler to drive changes to it in a more automated fashion, as follows:

• If during the command implementation, you find that the command needs to be made async, you can just change implemented interface in the handler from ICommandHandler > IAsyncCommandHandler and an analyzer + codefix will suggest changing the command from ICommand to IAsyncCommand in turn.
• If during the implementation, you find that you'd like to return a value, or perhaps even change the return type you previously had, an analyzer + codefix will suggest the relevant changes to the command too, setting/removing the return type as needed (by changing the ICommand or IAsyncCommand as needed, even removing the TResult entirely as needed.

Closes #38