Remove .extend(...) in favour of constructor

[mmkal]

Fixes #441

(by removing the method that had out of date docs!)

@papb no worries if you're not able to review this, but before doing the v3 release, I wanted to drop the .extend(...) method which is confusing, inflexible, and not really necessary.

This is a breaking change to the beta release. Users of .extend will have to change code like this:

const parent = new Umzug({ ... })

const umzug = parent.extend(
  migrations => migrations.slice().reverse()
)

To this:

const parent = new Umzug({ ... })

const umzug = new Umzug({
  ...parent.options,
  migrations: async ctx => (await parent.migrations(ctx)).slice().reverse(),
})

[github-actions]

Release v3.0.0-beta-cli.18 addresses this.