[m-rev.] for review: Add interface for joinable threads.

[Julien Fischer]

On Fri, 23 Feb 2024, Peter Wang wrote:

> Support joinable threads in C# and Java backends.
>
> library/thread.m:
>    Implement spawn_native_joinable and join_thread for C# and Java.
>
>    Rename the existing Java helper class RunGoal to RunGoalDetached.
>    Add RunGoalJoinable.
>
>    Rename the C# helper MercuryThread to RunGoalDetached, to match the
>    Java backend. Add RunGoalJoinable.
>
> java/runtime/MercuryThreadPool.java:
>    Replace submitExclusiveThread() method with createExclusiveThread(),
>    which returns the newly created thread, without starting it.

That'e fine.

Julien.