Make find's default predicate be identity

[Keno]

As discussed in #23005, the zeroness predicate is not uniquely defined. It seems odd to pick one for find. However, one really useful property of this is that it returns the true indices for boolean arrays, so you can do things like find(x .== 1), which we should preserve. I thus propose to make the default predicate be identity to retain the useful behavior for boolean arrays, but force the user to choose their predicate for numeric arrays.

[nalimilan]

+1

If we do that, then the Search & Find Julep should be updated to say "true values" rather than "non zeros" in a few places.