Attachment store for assistant2 (#11327)

This sets up a way for the user (or Zed) to _push_ context instead of
having the model retrieve it with a function. Our first use is the
contents of the current file.

<img width="399" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/198429a5-82af-4b82-86f6-cb961f10de5c">

<img width="393" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/cfb52444-723b-4fc1-bddc-57e1810c512b">

I heard the asst2 example was deleted in another branch so I deleted
that here too since we wanted the workspace access.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/assistant2/src/ui.rs

@@ -1,3 +1,4 @@
+mod active_file_button;
 mod chat_message;
 mod chat_notice;
 mod composer;
@@ -6,6 +7,7 @@ mod project_index_button;
 #[cfg(feature = "stories")]
 mod stories;
 
+pub use active_file_button::*;
 pub use chat_message::*;
 pub use chat_notice::*;
 pub use composer::*;