Fixes: #11687

Description

Refactors the List component's scroll growing functionality to use intersection observers with start/end markers instead of unreliable scroll height calculations and scroll container detection.

Background Problems

The existing scroll growing implementation had several issues:

• Issue [SF][List]: The load-more event should not be fired when re-render a List #10442
  False positive load-more events during rerender/content changes
• Issue <List> component growing="Scroll" no longer triggers scroll events after upgrade to 2.9.0+ #11687
  Failed detection when List is inside external scrollable containers
• Issue [List]: Growing list onLoadMore event not triggered in Firefox #11461
  Firefox issue solved by the new implementation as well

Solution

Replaces the previous approach with:

• Start marker: <span> element positioned at the beginning of the scroll container to track when user has scrolled down
• End marker: <span> element positioned at the end to trigger load-more events
• Viewport-based intersection observers: Two dedicated observers that detect marker visibility relative to the viewport
• State management: _startMarkerOutOfView property to ensure container was scrolled down

Test Coverage

• DOM presence tests for start/end markers
• Intersection observer functionality validation
• Edge case Readme links lead to 404 #1: Rerender scenarios that previously caused false events
• Edge case docs(playground): fix typo #2: External scrollable containers that previously failed detection