[labs/scoped-registry-mixin] Make compatible with subclassing

[sorvell]

Description

It looks like this mixin is not currently compatible with subclassing. A subclass should have it's own registry and be able to define or extend the elementDefinitions. Currently, this fails since the registry and elementDefinitions are only checked on the constructor.

To fix this, registry and elementDefinitions should be own property checks. In addition we need to provide some mechanism to extend elementDefinitions. We can either require users to explicitly add the super.elementDefinitions or do automatic composition as is done for properties.

[jpzwarte]

Just ran into this. Causing bugs here (fortunately not in production yet). Looking at using @open-wc/scoped-elements instead. This is the code in question:

  // MonthView already uses ScopedRegistryHost
  class CustomMonthView extends MonthView {
    static elementDefinitions = {
      ...MonthView.elementDefinitions,
      'dna-overlay-trigger': OverlayTrigger,
      'dna-tooltip': Tooltip
    };

    renderDay(day) {
      // custom render here
    }
  }

  customElements.define('custom-month-view', CustomMonthView);