feat: Support export default in migrations

[jrf0110]

Summary

This makes it so migrations can export their migration functions using ES module syntax in TS-Node or after running through webpack.

import { MigrationFunction } from 'contentful-migration'

const migrate: MigrationFunction = (migration) => {
  const dog = migration.createContentType('dog');
  const name = dog.createField('name');
  name.type('Symbol').required(true);
}

export default migrate

Motivation and Context

I had been receiving this error:

TypeError: migrationCreator is not a function
    at node_modules/contentful-migration/src/lib/migration-steps/index.ts:335:12

Saw some others as well:

• "migrationCreator is not a function" when using contentful-migration "runMigration" #124
• TypeError: migrationCreator is not a function #103

[dsbrianwebster]

@jrf0110 yes to this! Can we get a review from @Khaledgarbaya or someone at contentful?

[colinwirt]

Thanks @jrf0110 :)

@Khaledgarbaya @ruderngespra I'd love to see something like this supported to fit-in with fairly strict linting! Sometimes I find a project bans ignoring errors caused by this module syntax making it tough to work out how many levels of ignores are needed :/

It would be great to have this PR reviewed 🤓

[ItsDaveB]

This is still an issue, would be great to support this!