Decide where to define subcomponents

[woutermont]

Where do we define the tags for subcomponents in components? It seems weird that the root index file should know all used (sub)components. On the other hand, defining tags inside the (sub)component's constructor leads to already registered tag errors.

[woutermont]

Each component should define its own subcomponents in its constructor, or anywhere before the tag is used/rendered. We should add syntactic suggar to the SDK, but also provide an example code block to do so without. Start from the following but expand with extra checks if necessary:

if (!customElements.get(tag)) customElements.define(tag, module);