Add `--format=count` to `language core|plugin|theme list` commands

[ernilambar]

Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

In language core|plugin|theme list commands, --format does not support count.

Describe the solution you'd like

Introduce --format=count to language core|plugin|theme list commands. It would be handy and also in parity with our other list commands.
Eg: wp language core list --status=installed --format=count