Add support for nesting embeddables

This is a possible approach towards adding support for nesting embeddables (embeddables inside embeddables). I'm not sure whether this implementation is the best solution but I think it is something to start with. It seems to work flawlessly so far but I'm not sure if I missed something, so any feedback is welcome ;)
`private $embeddablesInheritance = [];` and then
```php
private function checkEmbeddablesInheritance($embeddableName, $looping = [])
{
    // do the recursion stuff here
}
```

Something like that. I suggest starting from a test, as it makes it much easier to write the recursion