t/96_interop.t: broke with LEHMANN/JSON-XS-3.01
t/96_interop.t: add JSON::PP test

JSON::PP cannot be used anymore, since JSON broke the JSON::PP API.
It is now named JSON::BackportPP but provides JSON::PP.
3.0101 release: fixed booleans for JSON 2.9 and JSON-XS-3.01 interop

add INTEROP with JSON and JSON::XS pod section.
add more interop tests

we just to know the JSON version beforehand so we can prepare our internal boolean representation