wip-new-core: Fix embedding for the JSON formatter output

In legacy_formatter calls to embed is passed on directly to the formatters. This means that they will arrive to the formatter "out of order" - before the formatter knows which the corresponding step is.

It is possible to make the JSON formatter able to handle such "out of order" calls to embed, but I think it is the responsibility of legacy_formatter not to make such a major change to the call sequence the formatter experience. Therefore the calls to embed from the step definitions is delayed in legacy_formatter until the formatter has been informed of the corresponding step.