Changing python-envs.terminal.autoActivationType back from shellStartupshould revert any known changes, provided they haven't been modified

[Tyriar]

Testing #226

Repro:

1. Set "python-envs.terminal.autoActivationType": "shellStartup"
2. Install
3. Set "python-envs.terminal.autoActivationType": "command", 🐛 the script additions and especially the registry edit is still in place

[karthiknadig]

This is expected. We don't undo, the reason being we can disable these by not including env variables. We could notify and expose the revert command.