Refactor float8 conversion logic.

This change completely rewrites the float8 conversion template to a more structured approach. It now explicitly handles special values (Inf/NaN), zero, unpacks and normalizes the input, calculates target parameters, applies rounding and shifting, renormalizes, and packs the result, including overflow and sign handling. The logic for handling subnormals and different exponent ranges between From and To types is integrated into this new flow.

PiperOrigin-RevId: 845859579

Author: majnemer