Simple syntax for batching migrations in `generate-script`

We have `inventory-report` for `ado2gh` and soon to be in `gei` for powerful batching and sorting flexibility. But alot of customers don't need that much flexibility, and just want a simple way to say "group my migrations into batches of 500 repos".

We could add an arg to `generate-script` something like `--batch-size` that would output several migration scripts, with 500 repos in each.

Customer could also accomplish the same thing with `inventory-report` and splitting up the csv's. But this could be a more convenient option for customers that want to do simple batching.