Can't query groups using api4

I opened the following Stack Exchange question: https://civicrm.stackexchange.com/questions/49150/trouble-with-cv-api-version-3-and-4/49151 and it looks like this is probably an issue with `cv`'s implementation itself.

The following error happens when trying to get (for example) the email address for every user in a given group:

```
$ cv api4 Contact.get +s email_primary +w 'groups:name = "Current Singers"'

In Api4Query.php line 348:

Illegal operator for groups
```

Reproduce the reported `cv api4 Contact.get +s email_primary +w 'groups:name = "Current Singers"'` query, then inspect `Api4Query.php` around line 348 where the illegal-operator error is reported. Done means the query can retrieve contact email addresses filtered by group without that error.

Written by the indexing model from the issue text.