Function fallback

fallback<T>(defaultValue: T, decoder: Decoder<T>): Decoder<T>
Decoder that falls back to a default value if the given decoder fails.
Type Parameters
- T
Parameters
- defaultValue: T
  The value to return if the decoder fails
- decoder: Decoder<T>
  The decoder to try first
Returns Decoder<T>
A decoder that returns the default value if the given decoder fails
Example
```
const numberOrZero = JsonDecoder.fallback(0, JsonDecoder.number());
numberOrZero.decode(42); // Ok<number>({value: 42})
numberOrZero.decode('not a number'); // Ok<number>({value: 0})
```