Add Decimal.prototype.closeTo(value, delta) for approximate equality checks

No built-in method exists to check approximate equality. Current workaround:

```typescript
a.minus(b).abs().lte(delta)
```

Proposed:

```typescript
Decimal(1.00000001).closeTo(Decimal(1), 0.00000001) // true
```

Feel free to change the function name, the point is to simplify approximation assertions :)