Provide a way to cut transaction "groups" and start a new transaction for a given migration file.

Hey

If there wasn't such a strong case to keep knex naming and behavior - familiarity and lots of training data, I'd call this something more explicit and perhaps go beyond boolean.

e.g. transactionMode?: 'never' | 'shared' | 'file-scoped', where "never" means no matter what, never runs this file's changes in a transaction, "shared" means (re)use or create a "global" transaction (the default mode), "file-scoped" which means always commit previous transaction group if available and start a transaction just for this file.

Not for now. Something to consider after this effort.

Originally posted by @igalklebanov in #1671