Add list-sets command

[keradus]

closes #5621
example: php php-cs-fixer list-sets --format json

[keradus]

@MaXal , would that fit your needs?
// part one: schema

[keradus]

@MaXal , would that fit your needs?
// part two: example json result

[keradus]

I was wondering about this special nesting level (let me call it "envelope") -> for now, we don't really need it.
Yet, we had cases in past, when Rule was expecting configuration like [foo, bar] and we couldn't extend it, causing a BC break to introduce {items => [foo, bar], otherOption: true}. let's be future ready

[coveralls]

Coverage decreased (-0.04%) to 91.897% when pulling ddf9c0e on keradus:list-sets into ab5358e on FriendsOfPHP:master.

[kubawerlos]

Why did the coverage dropped?

[keradus]

no clue

[MaXal]

Thank you a lot for your work! The result works great! I choose to implement a parser based on txt format since it's much easier to parse.

[keradus]

Thanks for the feedback, @MaXal !
In that case, I gonna merge the PR as-is.

Please, be warned that for JSON, the result is well-specified and you can rely on schema under BC promise. For txt output, the format of message can be modified at any PHP CS Fixer update.

[MaXal]

@keradus Thank you! I will take a look at JSON format then to make the support more bullet-proof!