Allow multiple image_sizes for regeneration

[apermo]

Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

I had the case that I needed to regenerate 2 image sizes, or rather 1 newly added and 1 existing image size.
To solve this I had to run wp media regenerate --image_size=new_image_size and wp media regenerate --image_size=updated_image_size

Since I am talking about a multisite with 6 sites that we want to update, and each has at least 10k attachments, running the two commands does not seem to be efficient.

Describe the solution you'd like

I would love to be able to run either:

Enhancing the existing --image_size to allow single or multiple image sizes)
wp media regenerate --image_size=new_image_size,updated_image_size

or

Introducing a new --image_sizes that will accept multiple, in order to keep the grammar correct.
wp media regenerate --image_sizes=new_image_size,updated_image_size

Note: I'm happy to propose a PR, I'd just need a heads up if you think it's a good addition and which of the two suggested routes to take. I might need help updating the related unit tests.