Add a way to test that a string represents a valid decimal value

In the case I need to use user input, I must first check if it's a valid value before creating a new Decimal instance, or else an error is thrown.

I was expecting a static function allowing to test the validity of a string, e.g.:
```js
Decimal.isValid(userInput)
```

Of course I could try/catch on an error, but an `isValid` static function is cleaner and simpler.
