Look if we can stop requiring the using to include extra decoders for decimal, int64, uint64, bigint

[MangelMaxime]

With Fable 4.0.5+ changes have been made to use the native bigint instead of a custom implementation.

This should reduce the impact on the bundle size and so allow us to not require the user to include these types via the extra decoder feature when using Auto coders.

The question is how to drive this change of API. Fable 3 is still really present now days, so I am not sure we can simply remove these types.

A solution could be to use FABLE_COMPILER_4 compiler directive and remove these functions when on Fable 4. This means that if the user is using Fable 4.0.0 he would have a bundle size increase but I think this could be a good middle ground.

However, the error will only be detected at compilation time by Fable as I don't think we can find a way to make it detected by the IDE directly.