Merge symplify/phpstan-extenisons here, as only few classes

[TomasVotruba]

Merges symplify/phpstan-extensions into this package (only a few classes), adding return type extensions and a Symplify error formatter.

All return type extensions are disabled by default and opt-in via parameters in your phpstan.neon:

parameters:
    symfonyReturnType: true   # Container::get($id) => $id type; Finder SplFileInfo::getRealPath() => string
    laravelReturnType: true   # Container::make($class) => $class type
    pathStrings: true         # getcwd()/dirname()/realpath() => always "string"

Parameter	Enables
symfonyReturnType	ContainerGetReturnTypeExtension, SplFileInfoTolerantReturnTypeExtension
laravelReturnType	LaravelContainerMakeTypeExtension
pathStrings	NativeFunctionReturnTypeExtension

The errorFormat: symplify formatter is registered unconditionally.