debugger: Improve appearance of session list for JavaScript debugging (#34322)

This PR updates the debugger panel's session list to be more useful in
some cases that are commonly hit when using the JavaScript adapter. We
make two adjustments, which only apply to JavaScript sessions:

- For a child session that's the only child of a root session, we
collapse it with its parent. This imitates what VS Code does in the
"call stack" view for JavaScript sessions.
- When a session has exactly one thread, we label the session with that
thread's name, instead of the session label provided by the DAP. VS Code
also makes this adjustment, which surfaces more useful information when
working with browser sessions.

Closes #33072 

Release Notes:

- debugger: Improved the appearance of JavaScript sessions in the debug
panel's session list.

---------

Co-authored-by: Julia <julia@zed.dev>
Co-authored-by: Remco Smits <djsmits12@gmail.com>

crates/dap/src/adapters.rs

@@ -378,6 +378,14 @@ pub trait DebugAdapter: 'static + Send + Sync {
     fn label_for_child_session(&self, _args: &StartDebuggingRequestArguments) -> Option<String> {
         None
     }
+
+    fn compact_child_session(&self) -> bool {
+        false
+    }
+
+    fn prefer_thread_name(&self) -> bool {
+        false
+    }
 }
 
 #[cfg(any(test, feature = "test-support"))]