Detect project for `edgedb instance` commands

[colinhacks]

Latest nightly seems not to detect projects for edgedb instance commands.

Running inside an initialized project:

$ edgedb
EdgeDB 2.0-dev.6625+f62dc75 (repl 1.2.0-dev.783+6690d4d)
Type \help for help, \quit to quit.
edgedb> \q
$ edgedb instance stop
edgedb error: Instance name argument is required, use '-I name'

[tailhook]

Yes. This was the plan: release a version that doesn't support that. So that edgedb instance stop $VARIABLE issues a warning (and errors out when $VARIABLE is empty). Wait for people to adjust scripts to use edgedb instance stop -I$VARIABLE and then detect project.

This is the second part of the #739

[colinhacks]

I think it's time to pull the trigger on this @tailhook