[WIP] fix: form array validation (for rewrite)

[maxime1992]

This PR aims to fix https://github.com/cloudnc/ngx-sub-form/issues/161 but unfortunately while it seems to be working just fine on the component hosting the FormArray, I've noticed that the error is not broadcasted up to the parent components.

Well, it actually is but one change detection cycle after :( And I'm unsure why.

I have a suspicion that it may be due to the fact that when the component is being initialised we're not re-sending the value up to the parent component which therefore doesn't call the validate hook? Unsure as I haven't found the time to dig much more into it.

I thought it'd be an easy fix but for now I want to get back on making sure we can release v6 asap and in order to do that we need to focus on the documentation :upside_down_face:.

If anyone wants to take a look why we're one tick too late please do :pray: :smile_cat:. Thanks!