Add a state property to Command

[gustavohenke]

Title.
The intention is that this will make it easier to tell exactly where the command is. My idea is that it can be used for #433, since it might be necessary to make a distinction between a command that was never started, and everything else.

Also marked it a breaking change because I removed the #exited property, as #state supersedes it.
Booleans do a poor job sometimes :)

Most LOC in this PR are coming from moving a few tests around.

[coveralls]

coverage: 99.2% (+0.003%) from 99.197%
when pulling 212d579 on command-state
into 0c64306 on main.

[gustavohenke]

happy new year 🥳