Add Decimal.zero and Decimal.one

Those cannot be constants since the number of fractional digits matters.

```ts
class Decimal {
  // Returns 0.0 with `fractionalDigits` fractional digits
  public static zero(fractionalDigits: number): Decimal;

  // Returns 1.0 with `fractionalDigits` fractional digits
  public static one(fractionalDigits: number): Decimal;
```