Feature: new "enter" command

## Motivation

When developing and AiiDAlab app, one typically needs to develop inside the AiiDAlab docker container.
Those using VSCode and such can configure connection to the container, but old-school guys like me using vim typically enter the container manually via

```console
docker exec -it -u jovyan aiidalab_default /bin/bash`.
```

It would be nice if I didn't have to type all of that each time. I have an alias for that, but I am also using multiple aiidalab-launch profiles so the alias approach does not work that well.

## Proposal

Create a new `enter` command that would essentially execute the command above, by using

```console
aiidalab-launch enter -p
```

The command should error out if the container is not running.