Refactor Timeline using the MVVM pattern and a virtual list component

[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 TimelinePanel
• Created a new TimelinePanelView component to replace the 'MessagePanel'

Changes

New component

Codebase migration

Test plan

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

Labs setting