Refactor slack config

[Yukaii]

https://github.com/Yukaii/zuzugo/blob/a2222fab08677e5f49a64248373f2bfe4f0d1fe7/lib/config.ts#L40-L49

Refactor to scoped config like:

slack: {
  incomingWebhook: process.env.INCOMING_WEBHOOK,
  ...
}