Ensure full flush on panic

[davidhewitt]

At the moment we have the ShutdownHandler type returned from logfire.configure(), which needs to have .shutdown() called at the end of the program to flush. Usually one does this at the end of main.

This won't run if the whole stack is unwound with a panic. We should probably use a guard pattern so that panics are fully exported before exiting the process.

[davidhewitt]

cc @cetra3