Rename `messages` parameter of `govukI18nAttributes`

[romaricpascal]

What

Rename the messages option of govukI18nAttributes, used to pass the different plural forms of a message.

The new name should be distinct enough from the option used when there's no plural form (message). A proposal would be messagePluralForms.

Why

It's current name is too close to message used when there's no plural forms, making it easy to make mistakes.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• [ ] The option name has been changed