ManualMigration class

A migration that uses callbacks for up/down operations

Example:

final migrations = [
  ManualMigration(
    version: 2,
    description: 'Seed initial data',
    onUp: (connection, schemaManager) async {
      await connection.execute(
        "INSERT INTO users (name, email) VALUES ('Admin', 'admin@example.com')",
      );
    },
    onDown: (connection, schemaManager) async {
      await connection.execute(
        "DELETE FROM users WHERE email = 'admin@example.com'",
      );
    },
  ),
];