Enum api that supports type builder api

[jasonkuhrt]

This is missing from the Nexus api:

enumType({
  name: 'Foobar',
  definition(t) {
    t.member('foobar')
  }
})

The reason this would be useful is for plugins that want to allow special enum features. For example Nexus Prisma could allow users to project enums from the DB level into the API level.