support core boolean values

[haarg]

There is currently a draft PR to add trackable boolean values to perl: Perl/perl5#19040

This PR adds support for those boolean values to JSON::PP.

The first commit recognizes the core boolean values and encodes them as JSON true/false values. If the core change goes ahead, this would definitely be wanted.

The second commit decodes JSON booleans into core perl boolean values. This would be nice, and is what we would want if perl had always had core boolean values. But it presents compatibility problems. Possibly we'll want a different solution, like a ->core_bools method as an easy alternative to ->boolean_values(!!0, !!1).

[haarg]

While it's not guaranteed that the core change will be merged, I figured it would be good to start discussion here about how to handle it.

[Grinnz]

I agree that the decode to core bools should be opt-in to start with.