Add a setting to force stepping to always run all threads.

Also allow ScriptedThreadPlans to set & get their StopOthers
state.

<rdar://problem/64229484>

Differential Revision: https://reviews.llvm.org/D85265

lldb/source/Target/Thread.cpp

@@ -1380,7 +1380,7 @@ lldb::ThreadPlanSP Thread::QueueThreadPlanForStepScripted(
 
   ThreadPlanSP thread_plan_sp(new ThreadPlanPython(*this, class_name, 
                                                    extra_args_impl));
-
+  thread_plan_sp->SetStopOthers(stop_other_threads);
   status = QueueThreadPlan(thread_plan_sp, abort_other_plans);
   return thread_plan_sp;
 }