Make DecoratedInterval the default

[Kolaru]

It has been discussed during the JuliaInterval call a couple of time, but without a clear consensus yet.

I think it would be good to have DecoratedInterval as the default. A new user would be warn when something weird happen, and that's probably what we want for them.

As they become more advanced, they could discover the faster, less safe Interval without too much trouble.

Of course that would mean, renaming DecoratedInterval to Interval, and Interval to BareInterval.

[OlivierHnt]

One advantage of doing this is that we can reintroduce conversion (implicit or explicit) with numbers for DecoratedInterval by adding a specific decoration.
While BareInterval would be more conservative to avoid confusion. E.g. bareinterval(1) + 1 could either error, or return a DecoratedInterval with the correct decoration 🤔