Allow more than one value map to the same enum value with `@JsonValue`

Currently, we can use `@JsonValue` in an enum to define what value maps to it.

```dart
enum CustomType {
@JsonValue('a')
valueA,
@JsonValue('b')
valueB,
@JsonValue('c')
valueC,
}
```

Now, I want to map more than one value to the same enum value. I cannot do this unless I write a custom `JsonConverter` for the enum.

This issue proposes that we have a way to tell `json_serializable` that we want more than one value to be mapped to our enum value.

For instance:
```dart
enum CustomType {
@JsonValue('valueA')
@JsonValue('a')
valueA,
@JsonValue('valueB')
@JsonValue('b')
valueB,
@JsonValue('valueC')
@JsonValue('c')
valueC,
}
```