Fall back to NNUE if classical evaluation is much lower than threshold

Fall back to NNUE if classical evaluation is much lower than threshold

The idea is that if classical eval returns a value much lower than the threshold of
its usage it most likely means that position isn't that simple
so we need the more precise NNUE evaluation.

passed STC:
https://tests.stockfishchess.org/tests/view/61bf3e7557a0d0f327c3c47a
LLR: 2.95 (-2.94,2.94) <0.00,2.50>
Total: 108072 W: 28007 L: 27604 D: 52461
Ptnml(0-2): 352, 12147, 28650, 12520, 367

passed LTC:
https://tests.stockfishchess.org/tests/view/61c0581657a0d0f327c3fa0c
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 155096 W: 40392 L: 39841 D: 74863
Ptnml(0-2): 88, 15983, 44843, 16558, 76

closes https://github.com/official-stockfish/Stockfish/pull/3869

bench 4310422