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/ui/src/components/icon.rs

@@ -184,6 +184,7 @@ pub enum IconName {
     Tab,
     Terminal,
     Trash,
+    TriangleRight,
     Update,
     WholeWord,
     XCircle,
@@ -303,6 +304,7 @@ impl IconName {
             IconName::Tab => "icons/tab.svg",
             IconName::Terminal => "icons/terminal.svg",
             IconName::Trash => "icons/trash.svg",
+            IconName::TriangleRight => "icons/triangle_right.svg",
             IconName::Update => "icons/update.svg",
             IconName::WholeWord => "icons/word_search.svg",
             IconName::XCircle => "icons/error.svg",