Add a way to do migrations

[rglover]

This would be a good last minute add before 1.0. It'd be helpful to have a simple, database-backed migration system that you can write up/down functions with. For example:

migrations.define({
  database: 'mongodb|postgresql',
  version: 'semver or date',
  upgrade: () => {},
  downgrade: () => {},
});

In the upgrade, you write the changes to move to, and in the downgrade, the changes you want to roll back. To run, just have the version pinned in node.app() like this:

node.app({
  migrations: {
    mongodb: '1.0.0',
    postgresql: '1.3.1',
  },
});

The above would allow us to support multiple databases and track which migration we should be at. Each respective database would get its own migrations collection/table that tells us this information. If it doesn't exist, we'd assume that whatever was past is both the first/current migration.

On startup of the app, we'd first handle any migration.define() calls and then check the migrations collection/table to see if we're already at the specified version. If not, we'd run the corresponding up function.

When "rolling back" to an older version, we'd need to take the passed version and diff against others in the database by their migratedAt/migrated_at field and decide which downgrade functions to call on the versions in the diff.

[rglover]

This should use a simple date like 2023-12-14 for the migration name instead of a semver (too messy).