[🚀esl-panel-group]: ability to get next/prev panel from the panel group

[ala-n]

As an ESL consumer, I want to be able to get the next or previous panel easily.

Initial use case: carousel-like behavior based on esl-panel-group element. No OOTB trigger for mentioned capabilities is required.

Scope:

The following JS API is proposed

Only cyclic behavior: $group.$nextPanel.show() $group.$prevPanel.show()