Add Core::Process::terminate_immediately() as the shared primitive
for terminating a process without running exit-time destructors. Use
_exit() on POSIX and TerminateProcess() on Windows.
Add an opt-in Core::Process spawn mode that asks supported kernels to
terminate the child when its parent exits. On Linux, use a fork/exec
path for this mode so the child can install PR_SET_PDEATHSIG before
execing the helper process. The existing posix_spawn path remains in
place for other process launches.
Opt LibWebView helper processes into this mode so WebContent,
WebWorker, and other browser children do not survive a crashed or
exited UI process.
Windows doesn't have a concept of zombie children, hence:
* `disown` not needed
* we need a process handle because otherwise if the process have ended
by the time `wait_for_termination` is called
its pid may be reassigned to other process
