Document missing features in labs.md

[psrpinto]

Document feature_new_layout_switcher flag to labs.md.

---

This PR currently has no changelog labels, so will not be included in changelogs.

A reviewer can add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is, or add Type: [enhancement/defect/task] to the description and I'll add them for you.

[psrpinto]

I noticed the following features are not in labs.md either:

• feature_thread
• feature_pseudonymous_analytics_opt_in

Should I add them too?

[turt2live]

Thanks! The other features can probably be added, though I think they're also recent enough where their authors might simply not be aware of this file - will ping them internally to open up PRs :)