…yntax (mytuny)

This PR was merged into the 6.2 branch.

Discussion
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[ExpressionLanguage] Add support for null coalescing syntax

| Q             | A
| ------------- | ---
| Branch?       | 6.1
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #45411, #21691
| License       | MIT
| Doc PR        | symfony/symfony-docs#16743

This is another waited feature for the syntax of the expression-language component. The [null-coalescing](https://wiki.php.net/rfc/isset_ternary) operator ``??`` becomes a need for variant programming needs these days.

Following my previous PR introducing the null-safe operator (#45795). I'm hereby introducing yet another essential operator to make the syntax even more complete.

The null-coalescing operator is a syntactic sugar for a common use of ternary in conjunction with ``isset()`` (in PHP) or equivalent in other languages. This is such a common use-case to the point that almost all majors programming syntax nowadays support a sort of a short-hand for that operation namely coalescing operator. Now it's time for the syntax of Expression-Language to do so!

Expressions like:

* ``foo.bar ?? 'default'``
* ``foo[3] ?? 'default'``
* ``foo.bar ?? foo['bar'] ?? 'default'``

will default to the expression in the right-hand-side of the ``??`` operator whenever the expression in the left-hand-side of it does not exist or it's ``null``. Note that this coalescing behavior can be chained and the validation logic takes decreasing priority from left to right.

Commits
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8e3c505 [ExpressionLanguage] Add support for null coalescing syntax