Canvas move <slot /> to shadow DOM <div />

[vatro]

While fixing #100, I realized that it doesn't matter where we set DOM target to with DOM-less components. If they're added via markup, the context (important) will be shared according to the hierarchy in the markup. If we create them manually (via e.g. new Mesh( ... ), the target property can be set to literally anything as it seems, important is the correct context (e.g. a child component has to be assigned the context of the parent component).

Since the only DOM elements we're using / generating are shadow DOM elements, it feels more consistent to put the <slot /> inside the shadow DOM <div />, so that the light DOM (the <canvas> element) is empty. Doing this doesn't break anything e.g. concerning interactivity etc. (checked).