Add the ability to follow the agent as it makes edits (#29839)

Nathan here: I also tacked on a bunch of UI refinement.

Release Notes:

- Introduced the ability to follow the agent around as it reads and
edits files.

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Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/icons/src/icons.rs

@@ -72,6 +72,7 @@ pub enum IconName {
     CopilotInit,
     Copy,
     CountdownTimer,
+    Crosshair,
     CursorIBeam,
     Dash,
     DatabaseZap,