Update for PHP 8.1+

[dereuromark]

First stab at #368

It will also require test harness to be fixed up for 8.1+

Declaration of PHP_CodeSniffer\Tests\TestSuite::run(?PHPUnit\Framework\TestResult $result = null) must be compatible with PHPUnit\Framework\TestSuite::run(?PHPUnit\Framework\TestResult $result = null): PHPUnit\Framework\TestResult

As well as (follow-ups?):

• Move tests into tests/ with proper autoload-dev instead of part of autoload

[dereuromark]

I enabled for now the full set of mult-line declarations, just for testing
We can redude this once we are ready for merging if it is too disruptive and only keep 1 of the 3 pairs

For now the pressing question is how to get the tests working on modern versions of dependencies.

[dereuromark]

Anyone any ideas on the blocker of tests getting green again?

[LordSimal]

Whats the state of this PR?

[dereuromark]

This looks like it needs a bit of rework now.
I would still like to see it added