chore: allow php-cs-fixer major version 3

[phil-davis]

Add the ability to use php-cs-fixer major version 3.

Add .php-cs-fixer.dist.php (the settings of php-cs-fixer for major version 3) and put the nullable_type_declaration check into it. This is the main reason for the PR - it makes sure that any of these that will emit deprecation messages in PHP 8.4, are found in CI.

Run cs-fixer on PHP 8.4.
Leave the old cs-fixer still running on PHP 7.1.
And run cs-fixer on every PHP version, to be sure. In CI, composer will choose whether to use major version 2 or 3 of php-cs-fixer.

The newer cs-fixer v3 makes various nice fixes that do not break on PHP 7.1.
The old cs-fixer v2 is happy with the changes, it does not try to change them back - that is nice.

This is only needed for this old release series that still has PHP 7.1 support. We can run both cs-fixer and phpstan (and phpunit) on PHP 7.1 through 8.4. I think that helps give confidence that the code works across that range of PHP versions.

Note: when I forward-port all this stuff to later major releases, I won't need to do all this tweaking because we move to supporting only PHP 7.4 and up.

Similar to:
sabre-io/xml#290
sabre-io/http#243
sabre-io/uri#117

Note: for php-cs-fixer v3 I also had to set:

'blank_line_between_import_groups' => false

Because v2 does it like that, and I couldn't find a way to force v2 to put a blank line between each group of "use" statements.

And define ordered_imports for php-cs-fixer v2, so that it works the way that v3 works.

That is fine for this 5.1 branch - in the later major release branches I won't need to fine-tune "odd" php-cs-fixer rules.
The "odd" rules were only relevant to examples/promise.php - that has ordinary use statements and a use function statement. So there was no "real" run-time code that was impacted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.72%. Comparing base (c832947) to head (162d4c6).
Report is 5 commits behind head on 5.1.

Additional details and impacted files

@@            Coverage Diff            @@
##                5.1     #132   +/-   ##
=========================================
  Coverage     99.72%   99.72%           
  Complexity      118      118           
=========================================
  Files             7        7           
  Lines           365      365           
=========================================
  Hits            364      364           
  Misses            1        1           

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[staabm]

Lgtm

[phil-davis]

Part of issue #123