center method
Finds the center (closest element).
Implementation
int center(List<T> list, V value, {int lo = 0, int? hi}) {
final hiValue = hi ?? list.length;
if (lo >= hiValue) return -1;
final i = left(list, value, lo: lo, hi: hiValue);
if (i == 0) return 0;
if (i == hiValue) return hiValue - 1;
return i - 1;
}