Replace node mutation by WeakMap

[Grsmto]

As discussed there it would be better to use a WeakMap to keep track of SimpleBar instances instead of mutating the nodes elements.

We also need to add a static helper to retrieve the SimpleBar instance (that will now sits at SimpleBar.instances.get(el)).

cc @andersk

[andersk]

Sounds good to me. Let's give the static helper a short name since Simplebar.instances.get(el).getScrollElement() is quite a mouthful. Maybe SimpleBar.of?

[Grsmto]

Yeah definitely.
I liked the SimpleBar.ofElement that you proposed before. Makes it clear what type of parameter it's expecting.

[andersk]

ofElement is more explicit, but I’m trying to imagine typical use cases like

SimpleBar.ofElement(document.getElementById("pane")).getScrollElement().addEventListener("scroll", () => {
    …
})

and wondering whether the extra word is actually worth the extra horizontal space, when there’s not really anything else that you could take the SimpleBar of. Your call though.