Add jl_active_task_stack to gcext API (#36823)
* Add jl_active_task_stack to gcext API
This is meant as a successor for `jl_task_stack_buffer`, which this
patch marks as deprecated in a comment.
* Let jl_active_task_stack also provide total stack range
This may be useful for debugging and other future applications, while
costing us little here; jl_active_task_stack is not supposed to be
used in hot paths either.
