Spec out how Commands can provide feedback to the user

[zadjii-msft]

I had left this as a future consideration. We probably need it in the MLP.

If an extension does something and wants to present feedback to the user, how do?

ex:

• when installing a new extension, provide a "Installing..." spinner

[zadjii-msft]

Car thoughts:

Right now, Command Palette is exclusively a client for objects that are hosted by COM servers in the extension processes. Could we have the host app also implement an object, then pass that back into the extension process? That way the extensions could have something to call back into us.

When we create a ICommandProvider, CmdPal can call ICommandProvider.Initialize(IExtensionHost). Then we could just have IExtensionHost.ShowMessage(message...) from inside the extension itself, or something like that.

I'd need to test if this is something that would work.