Add php-cs-fixer config and reformat

The formatting in this project has become something of a mess,
because it changed over time. Add a CS fixer config and reformat
to the desired st

Add php-cs-fixer config and reformat

The formatting in this project has become something of a mess,
because it changed over time. Add a CS fixer config and reformat
to the desired style, which is PSR-12, but with sane brace placement.

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Add support for ??= operator

Introduced in PHP 5.4, represented using an AssignOp\Coalesce node.