Investigate and implement nested slot component

[SuelenKarbivnychyy]

Right now, the application only supports slot as direct child of root_node span.
If slot is nested as child of another tag "as on the exempla below" the order will be messed up.

<pyml>
    <span>
        <div>
           <h2>This is just a test</h2>
        </div>
        <div>
            <slot></slot>
        </div>
    </span
</pyml>