feat: Add operator linebreak to psr12

[VincentLanglet]

Hi @Wirone

The rule is:
"Operators - when multiline - must always be at the beginning or at the end of the line."
cf https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/blob/v3.49.0/doc/rules/operator/operator_linebreak.rst

Which comply with
"Boolean operators between conditions MUST always be at the beginning or at the end of the line, not a mix of both."
from https://www.php-fig.org/psr/psr-12/#51-if-elseif-else

[coveralls]

coverage: 96.029%. remained the same
when pulling 16452d1 on VincentLanglet:onlyBoolean
into 438d5ea on PHP-CS-Fixer:master.

[keradus]

our fixer enforces that operator is always at beginning or always at end.

PSR expects that there is no mix, but doesn't enforce which variant to use