Introduce recent files ambient context for assistant (#11791)

<img width="1637" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/5aaec657-3499-42c9-9528-c83728f2a7a1">

Release Notes:

- Added a new ambient context feature that allows showing the model up
to three buffers (along with their diagnostics) that the user interacted
with recently.

---------

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

crates/ui/src/components/icon.rs

@@ -107,6 +107,7 @@ pub enum IconName {
     CopilotError,
     CopilotInit,
     Copy,
+    CountdownTimer,
     Dash,
     Delete,
     Disconnected,
@@ -221,6 +222,7 @@ impl IconName {
             IconName::CopilotError => "icons/copilot_error.svg",
             IconName::CopilotInit => "icons/copilot_init.svg",
             IconName::Copy => "icons/copy.svg",
+            IconName::CountdownTimer => "icons/countdown_timer.svg",
             IconName::Dash => "icons/dash.svg",
             IconName::Delete => "icons/delete.svg",
             IconName::Disconnected => "icons/disconnected.svg",