[Scala 3] Use implicit fns to enforce side-effect boundaries

[japgolly]

Side-effects are meant to appear in observations, and actions.

A potential problem is that observations then contain pure values (the actual data observations) and DOM references for use by actions. When Scala 3 comes out, implicit function types can be used to enforce side-effect capabilities such that the parts of the obs that are impure can be marked as such and be made callable only from actions and not assertions.