Make tracing easier

`--emit-timing` is proving invaluable, flamegraph is nice, but it remains at times painful to drill deeper. It would be nice to have additional tracing tools. @cmyr [noted](https://github.com/googlefonts/fontc/pull/642#discussion_r1424259839) [`xi_trace`](https://docs.rs/xi-trace/latest/xi_trace/) was useful in Xi and that there might be new shinier alternatives.

I'd really like the ability to easily get a flamegraph of a specific task. For example, give me the flamegraph of just glyph order or just kern instance tasks, etc.