Starting point for replacing the current timeline scroll bar with a virtualized list

[rbondesson]

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

Create labs setting and prepare for a new scroll panel

Summary

• Created a new labs setting, feature_new_timeline, for activating the new rendering path in MessagePanel
• Refactored MessagePanel and ScrollPanel to make a clean generic boundary for scroll panels

Changes

New component

• Added a new scroll panel component, ´TimelineScrollPanel` which will act as a wrapper for the coming MVVM component
• Duplicated the ScrollPanel unit-test suite for the TimelineScrollPanel for supporting the two rendering patchs

Codebase migration

• No screenshots have been updated for element-web playwright tests

Test plan

• Verify component renders correctly in the time line
• Run unit tests for affected components in element-web
• Run visual tests for affected components in element-web

Labs setting

[langleyd]

@rbondesson This seems to be working react virtuoso into the existing TImelinePanel and MessagePanel component tree, my suggestion was to replace them wholesale. So leave these old components largely untouched and build up the new timeline behind a labs flag. Evolving the existing one is an option with different tradeoffs but I think we could burn time on it.

This is what I had in mind -> #33042

[rbondesson]

Created a new starting point as @langleyd suggested. #33073