This change implements showing a summary of the current environment.
All you need to do is run
$ lumi env
and the current environment's information will be printed.
This makes it convenient to grab resource information that might be
required, for instance, to correlate with logs (e.g., lambda ARNs).
Eventually, as per pulumi/lumi#184, we want to print details about
all of the resources too.
This change is mostly just a rename of Moniker to URN. It does also
prefix resource URNs to have a standard URN namespace; in other words,
"urn🥥<name>", where <name> is the same as the prior Moniker.
This is a minor step that helps to prepare us for pulumi/coconut#109.
