Add mix task to undo latest codegen/delete latest migration

**Is your feature request related to a problem? Please describe.**

Codegen to generate a migration is really awesome, but sometimes you generate a migration and then you realize you missed something, so you want to delete that migration, make the change, and then generate it again. Currently that's a manual process - go find the migration, find the snapshots for all the relevant tables, delete them all, and do it again. It's pretty annoying!

**Describe the solution you'd like**

A task like `ash_postgres.delete_latest_migration` that will delete the last migration, and any snapshots that were created at the same time. No option to delete other migrations or anything like that - only the last one.

And it should probably confirm the name of the migration just to make sure!

**Describe alternatives you've considered**

You could just generate another migration with the added changes, but snapshot files are big and ugly and if you haven't even run the first migration, being able to undo/regenerate it seems like the smarter approach.

Start by locating the mix task that generates migrations and the code responsible for migration and table snapshot files. Trace how the latest migration and its associated snapshots are identified, then review existing task tests or migration-generation tests. Done means a task named ash_postgres.delete_latest_migration removes only the latest migration and its snapshots after confirming the migration name.

Written by the indexing model from the issue text.