the list of possible regions for cloud instances is not discoverable

EdgeDB CLI Version: 3.0.0-dev (`d763d26ead3fee56f53d4b4d4e72ed94b2e1c42c`, master branch as of 2023-05-11)

from `edgedb instance create --help`:

```
--region
For cloud instances: the region to create the instance in.
```

it will tell me if I get the region wrong:

```
> edgedb instance create --region test zackelan/test
This will create a new EdgeDB cloud instance with the following parameters:

Region: test
Server Version: 2.14
Does this look good? [y/n]
> y
edgedb error: "test" is not a valid EdgeDB cloud region
```

however, nothing in the help output will tell me what the valid regions are