Support for `make` jobserver protocol or system load-based job management

[lefou]

The idea is to reduce / avoid overscheduling due to running multiple Mill instances in parallel or concurrently to other build tools.

Here is a nice discussion of the issue and some solutions:

https://blogs.gentoo.org/mgorny/2025/11/30/one-jobserver-to-rule-them-all/

Technically, we should be able to dynamically adjust the maximum job could by replacing the fixed thread pool by a more dynamic one.

We could either try to limit the job count by watching the current system load, e.g. something analog to make -j8 -l9.

We could also try to support existing job server protocols, like the make jobserver protocol, which is already adopted by many build tools.