Implement PcntlForkJobRunner #4603

Job	Run time
Dependency Validation	13s
Coding Guidelines	37s
Unit Tests (ubuntu-latest, 8.3)	15s
Unit Tests (ubuntu-latest, 8.4)	17s
Unit Tests (ubuntu-latest, 8.5)	15s
Unit Tests (windows-latest, 8.3)	40s
Unit Tests (windows-latest, 8.5)	42s
Static Analysis	38s
Unit Tests (windows-latest, 8.4)	35s
End-to-End Tests (ubuntu-latest, 8.4)	1m 37s
End-to-End Tests (ubuntu-latest, 8.5)	1m 43s
End-to-End Tests (ubuntu-latest, 8.3)	1m 37s
End-to-End Tests (windows-latest, 8.3)	4m 16s
End-to-End Tests (windows-latest, 8.4)	3m 52s
End-to-End Tests (windows-latest, 8.5)	3m 54s
Build PHAR	0s
Code Coverage	0s
Test PHAR	0s
	21m 11s

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