feat: add ROUND_HALF_TO_EVEN flag for banker's rounding (fixes #251)

[jdymitarai]

Summary

Addresses feature request #251.

By default, \DoubleToStringConverter\ rounds halfway cases away from zero (towards +/-Infinity). However, modern C++ formatting standards (\std::to_chars, \printf, and IEEE 754 default rounding mode) use round-half-to-even (banker's rounding).

This PR introduces an opt-in flag \ROUND_HALF_TO_EVEN = 128\ to \DoubleToStringConverter::Flags\ without altering existing default behavior.

Key Changes

• Flag Definition: Added \DoubleToStringConverter::Flags::ROUND_HALF_TO_EVEN = 128\ with documentation.
• Propagation: Passed the flag down through \DoubleToAscii\ and \BignumDtoa.
• Rounding Logic: In \GenerateCountedDigits\ and \BignumToFixed, when the remainder is exact halfway (\PlusCompare == 0), the digit increments only if odd when
  ound_half_to_even\ is enabled.
• Zero Regression / 100% Backward Compatibility: Default conversions without this flag continue using the exact same fast-paths (\FastDtoa, \FastFixedDtoa) and existing rounding rules.
• Unit Tests: Added \TEST(RoundHalfToEven)\ in \ est/cctest/test-conversions.cc\ testing exact dyadic fractions (e.g. 0.5, 1.5, 2.5, 3.5, 4.5, -0.5, -1.5, 0.25, 0.75, 1.25, 1.75, 2.25, 0.125, 0.375, 0.625, 0.875) across \ToFixed, \ToPrecision, and \DoubleToAscii. All 9 test suites pass cleanly.

Closes #251.

[jdymitarai]

Closing for now to keep the PR queue focused. Thanks!