Add PHP 8 specific NullSafe operator mutators

[sidz]

This PR:

• Adds new NullSafeMethodCall and NullSafePropertyCall mutators
• Covered by tests
• Schema needs an update
• Doc PR: Update mutators.md with new NullSafe mutators site#202

NullSafeMethodCall Mutator:

- $object?->getObject()?->getName()
+ $object->getObject()?->getName()

and

- $object?->getObject()?->getName()
+ $object?->getObject()->getName()

NullSafePropertyCall Mutator:

- $object?->property?->name
+ $object->property?->name

and

- $object?->property?->name
+ $object?->property->name

I'm not sure how to exclude these mutators from the schema.json for case when Infection is executed on PHP < 8. Or maybe it won't be an issue as they won't be executed anyway due to PHP_VERSION_ID check.

[maks-rafalko]

👌

[maks-rafalko]

Cool, thank you @sidz