Add `locale` argument to `wp core multisite-install`

[ictbeheer]

Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

When creating a multisite install from scratch using wp core multisite-install it is not possible to specify to locale. For wp core install this feature was added in #133 but not for wp core multisite-install

Describe the solution you'd like

Allow locale as argument for wp core install-multisite. This should be enough to enable localized multisite installed as this argument is already handled by Core_Command::do_install() which is called by both wp core install and wp core multisite-install