Refine assistant config UI (#17871)

This PR does a little bit of a touch-up on the copywriting on the
assistant config UI. I had friends reporting to me that some of the
writing could be clearer, and hopefully, this goes into that direction!

Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -255,6 +255,7 @@ pub enum IconName {
     Slash,
     SlashSquare,
     Sliders,
+    SlidersVertical,
     Snip,
     Space,
     Sparkle,
@@ -284,6 +285,7 @@ pub enum IconName {
     XCircle,
     ZedAssistant,
     ZedAssistantFilled,
+    ZedXCopilot,
 }
 
 #[derive(IntoElement)]