Interpolation API is confusing

[cBournhonesque]

It's possible to call add_interpolation_fn without calling add_interpolation, which results in no interpolation running at all!

Also to enable visual interpolation on a component but without replicating that component, we have to call

app.add_interpolation::<Transform>(ComponentSyncMode::None);
app.add_interpolation_fn::<Transform>(TransformLinearInterpolation::lerp);

the 1st function creates some redundant systems and is just there to make sure that we register the component with mode None, instead of mode Full (which is the default when we call app.add_interpolation_fn::<Transform>(TransformLinearInterpolation::lerp);)