Function isExactly

isExactly<T>(value: T): Decoder<T>
Decoder that only accepts a specific value.
Type Parameters
- T
Parameters
- value: T
  The exact value to accept
Returns Decoder<T>
A decoder that only accepts the specified value
Example
```
const oneDecoder = JsonDecoder.isExactly(1);

oneDecoder.decode(1); // Ok<number>({value: 1})
oneDecoder.decode(2); // Err({error: '2 is not exactly 1'})
```