Fix: ensure child observation work properly with native & emulation for content projection

[bigopon]

At the moment, in unbind() of ChildObserverBinder, it's always unset the value via

this.viewModel[this.property] = null;

This works for native shadow DOM, because it's always ensured to retrieve the value again during bind(), as it's safe to do so. Originally, we didn't have this, and it was my attempt of trying to do proper cleanup, and it resulted in this subtle bug between native & emulation. To actually fix it, the above needs to be nested inside a native shadow DOM usage check:

if (this.useShadowDOM) {
  this.viewModel[this.property] = null;
}

For emulation, the content projection is done via an augmented view slot, and thus, can only work when content is actually projected through it, hence .bind()/.unbind() won't, and shouldn't affect it.

This PR also:

• guards change handler of child observation so that it's only called when the binder is bound.
• adds 11 integration tests for assertion different combination of if/repeat & parent/child to ensure that it works properly.

@EisenbergEffect

Close #675

[EisenbergEffect]

Looks like you are still working on this so just let me know when you feel that it's ready to merge. @bigopon I'm also fine with you doing the merge and taking on the lead role around au1 templating. You clearly know more than anyone else now. It's been a while since I looked into the depths of this area of code myself, so I trust you more than I trust myself 😄

[bigopon]

@EisenbergEffect All done 2 commits ago. Just this morning I realize the way linting process bails at the moment could easily lead to 2 - 3 follow up commits so I changed it to aggregating all linting errors before exiting. Ready to go any time. With regards to taking the lead, sure I'll take care of that. Though it's good to have someone looking at this fresh.

Mostly it's the change from

  unbind() {
    if (this.viewHost.__childObserver__) {
      this.viewHost.__childObserver__.disconnect();
      this.viewHost.__childObserver__ = null;
      this.viewModel[this.property] = null;
    }
  }

to:

  unbind() {
    if (has observer) {
      // 1. remove this binder
      // 2. check if there's no binder on the observer, and remove the observer itself if 0 binder
      // 3. check if it's safe to unassign the value in the view model, via useShadowDOM property
    }
  }