Add data migration routines#486

When writing a reply to this forum question I had the idea having this feature in Kirby:

Kirby data migrations

Similar to how you do migrations in a database system (e.g. compare Laravel’s up() and down() methods we could have a similar or the same routine to add and remove data structures in a file-based system.

It would be useful to add new fields to blueprints with defaults other than null / false / not set yet.

Now that we also have the Kirby CLI back, this would make a useful addition in my opinion. What do you all think about it?