[config] nest parameters under symplify vendor, as convention

[TomasVotruba]

Nest all opt-in parameters under a single symplify: vendor key, as is the PHPStan convention for extensions. Flat parameters are kept as backward-compatible aliases.

Config nesting change

Before, each rule set was toggled by a top-level flat parameter:

parameters:
    mocks: true
    ctor: true
    symfonyReturnType: true

Now the preferred, clearer form is nested under symplify::

parameters:
    symplify:
        mocks: true
        ctor: true
        symfonyReturnType: true
        laravelReturnType: true
        pathStrings: true