Generate job & process exit events for background jobs

[Baspar]

Description

Allow for --on-process-exit and --on-job-exit to be called inside of an event handler, as long as it is from a background job

(partially) Fixes issue #8766

Reproducing the issue

function __test_background_job_exit_event --on-variable dummy_var
  sleep 1&
  function callback --on-process-exit (jobs --last --pid)
    echo -n "Callback called"
  end
end

set dummy_var ""
# after 1s, "Callback called" will be printed on the screen

[ridiculousfish]

Merged as 2410e27 - thank you!