[Feature]: Add Ouput log

[jmatsushita]

Is your feature request related to a problem? Please describe. When switching to an environment that might have problems building, the "Applying environment..." status bar spinner can go forever without the ability to look at an output log to troubleshoot the problem.

Describe the solution you'd like Other extensions use the Output tab in the Panel to log their output (for instance the Haskell extension uses it to display Haskell Language Server launch info).

Describe alternatives you've considered Maybe a log file setting?

[samuela]

Just ran into this issue as well. It would really be great to stop the spinner and present an error message when nix-shell fails. And having logs in the Output pane would be a nice addition whether or not the nix-shell works correctly or not.

[arrterian]

Done #84 by @nh2