Reconsider RecurrenceSet::isInfinite method name

[dmfs]

In many cases it's probably easier to deal with when we use the simpler form

boolean isFinite();

The only real use case within lib-recur itself is in LastInstance, which only has a result if the recurrence set is finite.
At present this requires a negation, however, along with dmfs/jems#385 this could be simplified from

new Restrained<>(() -> !recurrenceSet.isInfinite(), new Last<>(recurrenceSet))

to

new If<>(recurrenceSet::isFinite, new Last<>(recurrenceSet))

reducing the cognitive load this needs to be understood.

Initially, to avoid a breaking change, the isFinite method could be added as a default method. In a second step we could swap this to make isInfinite the (deprecated) default method and in a third step we would just drop it entirely.