Add PHP 8.1 support

[Th3Mouk]

Hello ! This package is used in Symplify packages, therefore in many test suites.
I'm working on a new project and I want to profit of new PHP 8.1 features.

I didn't find any issue on this topic, and the release of PHP 8.1 is approching (november 2021).

How do you see things in this regard ? Which major version of nette/utils should support this version (for the base branch of the PR) ?
Can you remind us what was blocking at the time to have put a strong constraint?

I will try to help you, not using the lib directly myself.

Thanks for your time

[Th3Mouk]

TY @dg