key merge strategy

[pi0]

trafficstars

(this is a primary draft to explain the idea)

Related: #138, #87

Sometime default defu merging strategy is not ideal. We could allow a custom defu merger but it is not initiative.

I was thinking to try with an object syntax like this:

{
  mergeStrategy: {
    modules: (a, b) => a.concat(b)
  }
}

(it could be backported to defu)