Debugger: Implement remainder of #12729.

UserInterfaceListener:
- Add request hook for writing a core file. Implement in TeamDebugger.

Jobs/WriteCoreFileJob:
- Add new job to actually dispatch the core file request via the debugger
interface.

Team{::Listener}
- Add listener event + hook for notifications when a core file gets written.
Implement in CLI.

CliContext:
- Add event flag for core file changed.

CommandLineUserInterface:
- Add 'write-core' command. This optionally takes a path to write the core to,
otherwise one is automatically generated by, similarly to debug reports. As
such, one can now generate cores for things like app_server and registrar
crashes if desired, in addition to reports.