Revamp Scrollspy using Intersection observer

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 (closed) closes #33943 (closed) closes #28638 (closed) closes #35796 (closed) closes #28807 (closed) may fix #35628 (closed)