feat: Add an easier way to debug

[theofidry]

I would like to make cases like PHP-CS-Fixer/PHP-CS-Fixer#8129 easier to debug. There is 3 things to check:

• If each respective finder behaves as expected, this is the job of the diagnose.php.
• Which finder result got picked: the introduce CpuCoreFinder::trace() should answer that.
• A way to debug CpuCoreFinder::getAvailableForParallelisation()*

*: this is more tricky as:

• the result of the cores count may be memoized
• it depends on the injected parameters
• we likely want details of the calculation

What I am thinking is, since #127 is not released yet, I could change the signature of ::getAvailableForParallelisation() to return a result value object which could capture the resolved parameters to be able to figure out with which values the calculation was done.

What I am unsure of, is if it should contain the trace introduced there too, and if done it likely would need to be lazily executed as this would execute several additional system calls.

[theofidry]

/cc @Wirone in case you're interested in this