Revamp Scrollspy using Intersection observer (#33421)

* Revamp scrollspy to use IntersectionObserver

* Add smooth scroll support

* Update scrollspy.js/md

* move functionality to method

* Update scrollspy.js

* Add SmoothScroll to docs example

* Refactor Using `Maps` and smaller methods

* Update scrollspy.md/js

* Update scrollspy.spec.js

* Support backwards compatibility

* minor optimizations

* Merge activation functionality

* Update scrollspy.md

* Update scrollspy.js

* Rewording some of the documentation changes

* Update scrollspy.js

* Update scrollspy.md

* tweaking calculation functionality & drop text that suggests, to deactivate target when wrapper is not visible

* tweak calculation

* Fix lint

* Support scrollspy in body & tests

* change doc example to a more valid solution

Co-authored-by: XhmikosR <xhmikosr@gmail.com>
Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

.bundlewatch.config.json

@@ -34,7 +34,7 @@
     },
     {
       "path": "./dist/js/bootstrap.bundle.js",
-      "maxSize": "42.5 kB"
+      "maxSize": "42.75 kB"
     },
     {
       "path": "./dist/js/bootstrap.bundle.min.js",