Allow config to define thread count (4.x)

[M1ke]

Applying the same commits from #7442 to the 4.x branch

[M1ke]

As mentioned here (#7442 (comment)) if this gets merged to 4.x I am happy to resolve any conflicts with master, but I believe it should fit in directly.

[M1ke]

I can see that the check ci/circleci: test-with-real-projects fails and it's hard to see what in this branch would cause this - nothing in this work would change how Psalm carries out its analysis, and it will only apply the thread limit if the config file is modified.

[orklah]

failures in real projects are indeed unrelated :)

[orklah]

Thanks!