Also, factors out the fluent building APIs from IntoElement into their
own traits.

Also makes the project panel context menu initialization fluent:

<img width="1328" alt="Screenshot 2024-01-18 at 3 33 45 PM"
src="https://github.com/zed-industries/zed/assets/2280405/3468b6f2-07f0-48cf-bec1-ac0379333209">

Release Notes:

- Fixed pop in when right clicking on the project panel.