⚡ Make TaperedEvaluationTerm fields constant

[eduherminio]

Make TaperedEvaluationTerm constant using source generators, without having to hardcode the Utils.Pack everywhere formula.

I don't expect this to bring any improvement (or regression) per se, it's more of a proof of concept previous to the compile time calculation of TaperedEvaluationTermByXYZ (int[]), which could take advantage of .NET 8 perf improvements in constant array initialization via ReadOnlySpan<T>.

See experiment/source-generators branch for reference, and eduherminio/SourceGenerators_PoC.

Test  | perf/sourcegen-const-TaperedEvaluationTerm
Elo   | 1.15 +- 2.51 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 2.90 (-2.25, 2.89) [-3.00, 1.00]
Games | 31748: +8911 -8806 =14031
Penta | [767, 3683, 6893, 3740, 791]
https://openbench.lynx-chess.com/test/948/