New default ruleset / @auto

[keradus]

Right now we have @PSR-CS / @PER12 as default.
I would suggest a new one, like @auto, that would check composer.json and expand into:

[
    '@PHP{xy}Migration' => true,
    '@PHPUnit{xy}Migration' => true,
    '@PER-CS' => true,
]

(if risky fixers allowed, auto-add :risky version of sets)

And then, when someone would like to configure their project in details, they can build on top, eg:

[
    '@auto' => true,
    '@Symfony' => true,
]

[mvorisek]

@PHPAutoMigration and @PHPUnitAutoMigration would be my preferred choise.

[keradus]

I believe it's valid to add them, but I would still go with @auto on top. This could prevent a need to use config file / cli params for base usage.

[Wirone]

In this context it would be even better to have #7264 implemented, so final configuration can be described and it would be clear that some of the rules come from "auto" rules. In terms of the idea itself: looks OK. If I understand correctly it would determine PHP version (minimal supported?) and PHPUnit (minimal required?) only?