struct AsyncBacktraceUpdate

This custom event allows DAP clients to construct a custom async-backtrace UI that mirrors the

standard multi-threaded stacktrace UI.

Just as the standard stacktrace UI shows a list of threads, each with their own stack of frames

underneath, this custom event makes it possible for clients to show a list of threads, each with

their own tree of async-backtrace tasks underneath.

Here is how thread lifecycle events impact the `AsyncBacktraceUpdate` event:

- When a thread is created: An empty `tasks` array is sent.

- When a thread is stopped: An async-backtrace is collected and used to populate `tasks`.

- When a thread is resumed: An empty `tasks` array is sent.

- When a thread is destroyed: The `tasks` property is omitted from the event.

NOTE: This needs to be backwards compatible and in sync with `AsyncBacktraceUpdateData` here:

https://fuchsia.googlesource.com/vscode-plugins/+/refs/heads/main/src/zxdb/async_backtrace.ts