New revision of the Assistant Panel (#10870)

This is a crate only addition of a new version of the AssistantPanel.
We'll be putting this behind a feature flag while we iron out the new
experience.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Conrad Irwin <conrad@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Antonio Scandurra <antonio@zed.dev>
Co-authored-by: Nate Butler <nate@zed.dev>
Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Max <max@zed.dev>

crates/ui/src/components.rs

@@ -1,6 +1,7 @@
 mod avatar;
 mod button;
 mod checkbox;
+mod collapsible_container;
 mod context_menu;
 mod disclosure;
 mod divider;
@@ -25,6 +26,7 @@ mod stories;
 pub use avatar::*;
 pub use button::*;
 pub use checkbox::*;
+pub use collapsible_container::*;
 pub use context_menu::*;
 pub use disclosure::*;
 pub use divider::*;