Allow mini-migrations from plugins

[nataliecarey]

What

"Mini migrations" are being introduced by a different ticket: #2266

This helps users in one particular state caused by upgrading to v13 of the prototype kit. Similar changes might be needed as users upgrade to new versions of plugins,

One example is that we're asking @x-govuk/govuk-prototype-filters to namespace their filters and deprecate/remove the existing names for their filters. If they do this and a user upgrades to their latest version the user's prototype will break because it references filters that don't exist.

The plugin developer should be able to specify a "mini migration", we should provide them with a mechanism to look through the user's code and detect the outdated state. If they detect that state we should show a "mini migration" button allowing the user to run the "mini migration", the behaviour of this "mini migration" should also come from the plugin.

As part of this we should move our own "mini migration(s)" to use that mechanism.

Why

So that plugin developers have the tools they need to support users in updating.

Who needs to work on this

• Developer
• Designer (for the interface decision)

Who needs to review this

• Developer
• Designer (for the interface decision)

Done when

• Plugins can provide "mini migrations"
• Any "mini migrations" we have are moved into our govuk-prototype-kit.config.json file