Rework context insertion UX (#12360)

- Confirming a completion now runs the command immediately
- Hitting `enter` on a line with a command now runs it
- The output of commands gets folded away and replaced with a custom
placeholder
- Eliminated ambient context

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b1927a45-52d6-4634-acc9-2ee539c1d89a">

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -305,6 +305,7 @@ impl MessageEditor {
                     documentation: None,
                     server_id: LanguageServerId(0), // TODO: Make this optional or something?
                     lsp_completion: Default::default(), // TODO: Make this optional or something?
+                    confirm: None,
                 }
             })
             .collect()