debugger: Update New Session Modal (#30018)

This PR simplifies the new session modal by flattening its three modes
and updating the UI to be less noisy. The new UI also defaults to the
Debug Scenario Picker, and allows users to save debug scenarios created
in the UI to the active worktree's .zed/debug.json file.


Release Notes:

- N/A

crates/debugger_ui/src/tests/attach_modal.rs

@@ -103,7 +103,7 @@ async fn test_show_attach_modal_and_select_process(
         });
     let attach_modal = workspace
         .update(cx, |workspace, window, cx| {
-            let workspace_handle = cx.entity();
+            let workspace_handle = cx.weak_entity();
             workspace.toggle_modal(window, cx, |window, cx| {
                 AttachModal::with_processes(
                     workspace_handle,