Disable IIR and quiet shallow pruning in PV lines
Disable IIR and quiet shallow pruning in nodes that are part of the PV
line from the previous search iteration. This is done by storing the PV
in Worker and adding a followPV flag to Stack. The new flag may also be
used in other pruning techniques, allowing further improvement.
Passed STC:
```
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 63776 W: 16679 L: 16323 D: 30774
Ptnml(0-2): 169, 7441, 16377, 7667, 234
```
https://tests.stockfishchess.org/tests/view/69aad1a7d0f65de95886e289
Passed LTC:
```
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 87342 W: 22421 L: 22000 D: 42921
Ptnml(0-2): 40, 9443, 24299, 9834, 55
```
https://tests.stockfishchess.org/tests/view/69ab5993cb31ee884aed629d
closes https://github.com/official-stockfish/Stockfish/pull/6656
Bench: 2434614