This would allow an embedder to suspend the current Janet fiber
via an external event like a signal, other thread, or really anything.
This is a useful primitive for custom schedulers that would call
janet_interpreter_interupt periodically (say, in an interval with SIG_ALRM),
do some work, and then use janet_continue on the janet_root_fiber, or
for embedding into other soft-realtime applications like a game. To say,
only allow about 5ms per frame of interpreter time.
