Gets the index for the current thread when executing a job, otherwise 0.
When multiple threads are executing jobs, no two threads have the same index. The range of this property is [0, Unity.Jobs.LowLevel.Unsafe.JobsUtility.ThreadIndexCount).
The value returned when used from within a job is the same as the value stored in job members decorated with [Unity.Collections.LowLevel.Unsafe.NativeSetThreadIndexAttribute].
SA: Unity.Jobs.LowLevel.Unsafe.JobsUtility.ThreadIndexCount