[FEATURE]: Migration Squashing

### Feature hasn't been suggested before.

- [x] I have verified this feature I'm about to request hasn't been suggested before.

### Describe the enhancement you want to request

Sometimes, during development on a feature branch, we end up making multiple migrations as we iterate on schema design. However, this results in many migration files rather than one per PR.

It would be helpful if the drizzle-kit CLI had a "squash" command that allowed us to combine multiple migrations before merging them with our main branch.