Revamp Scrollspy using Intersection observer

[GeoSot]

Based on #33015.

Seems operational on the preview scrollspy page.

Needs:

• Documentation
• tweak tests

Advantages:

• No need of relative/overflowing wrappers etc
• Keeps offset config as deprecated, for backwards compatibility
• Emits "activate" events in the same element that was initialized on, discarding the previous incompatibility where in case of body initialization, event was emitting to window

Disadvantages:

• The method option is ignored now
• all first observables, get the active class on first render

closes #35727
closes #33943
closes #28638
closes #35796
closes #28807
may fix #35628

[GeoSot]

After three months, I think, I have something to show 😃

[XhmikosR]

It seems there's some funky thing going on in https://deploy-preview-33421--twbs-bootstrap.netlify.app/docs/5.0/components/scrollspy/#example-with-nested-nav item 3.1: if I scroll to 3.2 and then back up, 3.1 isn't highlighted.

It might need some tweaking with the stepping or whatever it's called :) Also, not sure what the TODO means? If the PR isn't ready yet, let's mark it as draft.

[GeoSot]

Unfortunately it is the same problem as now. Two visible elements in wrapper. Only the upper can be highlighted.
Tried some tricks, like visible percentage, but would lead to other problems if par example the element is higher than its overflowing wrapper 😞

[ffoodd]

On the feature side, it does not highlight exactly at the same moment than our previous version, but this one feels more natural to me.

Regarding the nested items talked about previously, I can confirm it already does the same in our current release and even worse: this PR does highlight the 3-1 item very quickly, what our current release doesn't do.

Si I'd go for it :)