nix-env-selector Add logging to VS Code output channel

Create a VS Code output channel during init
Log basic operations and error messages

Status	Type	Config Change
Ready	Feature	No

Problem

As mentioned in #59 , it would be useful to have the extension log to the Output window, for example to help troubleshoot Nix errors.

Solution

This creates a VS Code Output Channel during init, to which log messages can be appended.

I add logs for these operations:

Load config
Select/hit env
Run a sync/async command

Example output in the Nix Env Selector channel, including the error referenced in #59:

Initializing config...
Loaded config: {:workspace-root "/home/dan/test_nix_env", :nix-file "/home/dan/test_nix_env/env.nix", :suggest-nix? false, :nix-packages #js [], :nix-args nil, :nix-shell-path "/nix/store/hapw7q1fkjxvprnkcgw9ppczavg4daj2-nix-2.4/bin/nix-shell"}
Running command synchronously: /nix/store/hapw7q1fkjxvprnkcgw9ppczavg4daj2-nix-2.4/bin/nix-shell "/home/dan/test_nix_env/env.nix" --run export
Error applying environment: Error: Command failed: /nix/store/hapw7q1fkjxvprnkcgw9ppczavg4daj2-nix-2.4/bin/nix-shell "/home/dan/test_nix_env/env.nix" --run export
error: attribute 'npm' missing

       at /home/dan/test_nix_env/env.nix:5:9:

            4|     buildInputs = [
            5|         pkgs.npm
             |         ^
            6|     ];
(use '--show-trace' to show detailed location information)

Running action: Select environment
Running action: Hit environment
Selected to disable Nix environment
Selected Nix file: /home/dan/test_nix_env/env.nix
Loading env in path: /home/dan/test_nix_env/env.nix
Running command asynchronously: /nix/store/hapw7q1fkjxvprnkcgw9ppczavg4daj2-nix-2.4/bin/nix-shell "/home/dan/test_nix_env/env.nix" --run export
Error applying environment: error: attribute 'npm' missing

       at /home/dan/test_nix_env/env.nix:5:9:

            4|     buildInputs = [
            5|         pkgs.npm
             |         ^
            6|     ];
(use '--show-trace' to show detailed location information)