[POC] Add Expectation::isJson() to verify json string and decode

[kbond]

Per a slack discussion with @nikophil.

Assert::that('[4, 5, 6]')
->isJson() // json_decode's the current value and starts a new expectation with this
->contains(5)
;
Assert::that('5')
->isJson() ->is(5)
->isGreaterThan(4)
;

[kbond]

Not sure we want to do this but wanted to capture our conversation with a POC. We could go even further and move the Json class from zenstruck/browser here (and have Expectation::isJson(): Json).

[nikophil]

not sure of the name: it sounds like it's just an expectation, but it does more... decodeJson() ?

[kbond]

Yeah, this tripped me up as well. Technically it is also an expectation but is not clear that it changes the underlying data.

I chose this purely because the api reads better: "assert that value is json" vs "assert that value decode json". I definitely probably get too hung up on these semantics 😄.