CLI: Add defaults for `verdi computer [setup|configure]`

[GeigerJ2]

During verdi computer setup, I'd suggest the following defaults:

• Transport plugin -> core.ssh_async
• Scheduler plugin -> core.slurm

The rest of the options should be straightforward enough for new users to figure out what to enter. Only "Escape CLI arguments in double quotes" has caused confusion. Maybe this can be renamed?

The defaults could be shown in the CLI behind the option in sqare brackets. If one just presses enter, the default gets selected. The setup would look like:

❯ verdi computer setup
Report: enter ? for help.
Report: enter ! to ignore the default (in square brackets) and set no value.
Computer label: ...
Hostname: ...
Description []: ...
Transport plugin [core.ssh_async]: 
Scheduler plugin [core.slurm]: 
Shebang line (first line of each script, starting with #!) [#!/bin/bash]: ...
Work directory on the computer [/scratch/{username}/aiida/]: ...
Mpirun command [mpirun -np {tot_num_mpiprocs}]: ...
Default number of CPUs per machine: ...
Default amount of memory per machine (kB).: ...
Escape CLI arguments in double quotes [y/N]: ...

Setting core.ssh_async as the default also has the added benefit that almost nothing needs to be entered in verdi computer configure, and chances are high people already have the remote computer setup on their workstation via OpenSSH.