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.

---------

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

crates/editor/src/editor_tests.rs

@@ -12650,7 +12650,7 @@ async fn test_following_with_multiple_excerpts(cx: &mut TestAppContext) {
             Editor::from_state_proto(
                 workspace_entity,
                 ViewId {
-                    creator: Default::default(),
+                    creator: CollaboratorId::PeerId(PeerId::default()),
                     id: 0,
                 },
                 &mut state_message,
@@ -12737,7 +12737,7 @@ async fn test_following_with_multiple_excerpts(cx: &mut TestAppContext) {
             Editor::from_state_proto(
                 workspace_entity,
                 ViewId {
-                    creator: Default::default(),
+                    creator: CollaboratorId::PeerId(PeerId::default()),
                     id: 0,
                 },
                 &mut state_message,