Theorem LaurentSeries.exists_ratFunc_val_lt

feat(WithZero): `exp : ℤ → ℤᵐ⁰`, `log : ℤᵐ⁰ → ℤ` (#24977) …

feat(RingTheory/LaurentSeries): finishes the proof that LaurentSeries are the X-adic completion of Rational Functions (#18346) …