Add --json output

[TomasVotruba]

Useful for piping to other tools 😉

vendor/bin/phpstan-bodyscan --json

↓

[
    {
        "level": 0,
        "error_count": 0
    },
    {
        "level": 1,
        "error_count": 5
    },
    {
        "level": 2,
        "error_count": 25
    }
]