Rewrite using Vanilla JS

[zipper]

The goal is to rewrite the whole plugin using vanilla JS only.

[zipper]

Almost there:

• Github pages are not up to date. Document changes - different usage without jQuery, fx option removed, attribute hidden is always used, etc.
• Constructor should be able to recieve NodeListOf<HTMLElement> or HTMLElement (now only works with NodeListOf).
• Remove dist folder from github. Build should be done via github actions.
• Reconsider className in CollapsableItem. Maybe add an option for this class?