Do a major cleanup in Collapse.js

[GeoSot]

• streamline _getConfig & interface
• Remove redundant check on data-toggle click, as it was assuming that trigger element would own separate config than the collapse itself (wasn't documented)
• Use helper function to check show
• Remove helper for isTransitioning
• Use forEach for all iterations
• refactor privatef unction to use it in more cases
• initialize array variable properly
• Simplify check for children and make it generic
• remove duplicated Selector.find
• keep parent only as element
• simplify initialization on data-toggle click
• transfer interface inside jQueryInterface

NOTE for reviewers:

Better review it, commit by commit. It will help you with the proper message and will guide you with sanity to follow the logic

Closes #34408

[alecpl]

Why introduce a BC break with "keep parent only as element"? Some other components support string|element items.

[GeoSot]

Why introduce a BC break with "keep parent only as element"? Some other components support string|element items.

Hmmm I don't think it is a breaking change, as the tests hasn't been change at all.

The reality is, that now, the component acts as chameleon, parent may be string, may be element may be false empty string, to loose approach that causes many checks inside, and typeCheck passes as empty string is still a string

with this MR, getElement is used to initialize parent, so it still supports strings. However I am not sure if it has to be written on config object somehow

[alecpl]

Why introduce a BC break with "keep parent only as element"? Some other components support string|element items.

Hmmm I don't think it is a breaking change, as the tests hasn't been change at all.

That means the tests aren't complete. This is a public API: https://getbootstrap.com/docs/5.0/components/collapse/#options

[GeoSot]

May I don't express it well. It still accepts string selectors and jQuery objects. In its internals it keeps the vanilla js element and only.

And the real check is if the element exists or not. (right?)

[alecpl]

I may misinterpreted the code, I don't know it well. If these type defs are for internal use only, then there's no problem.

[mdo]

Can't speak to the JS changes, but clicking around, things look good inn the docs!

/cc @twbs/js-review for proper review

[mdo]

Ping @twbs/js-review for this one :)