feat: add support for converting is_null ternaries to null coalescing operator

[Soean]

This pull request extends the TernaryToNullCoalescingRector to support converting ternary expressions using is_null and its negation to the null coalescing operator (??).

Fixes: rectorphp/rector#9648

[samsonasik]

This can wait for PR:

• [Php70] Keep right parentheses on TernaryToNullCoalescingRector #7889

to be merged first and need sync to add check parentheses fixture on the right side of ternary.

[samsonasik]

Please rebase latest main branch, add fixture like in PR:

• [Php70] Keep right parentheses on TernaryToNullCoalescingRector #7889

and verify enhancement like in there to cover parentheses in the else/right part.

if ($ternary->else instanceof Ternary && $this->isTernaryParenthesized($this->file, $ternary->cond, $ternary)) {
     $ternary->else->setAttribute(AttributeKey::WRAPPED_IN_PARENTHESES, true);
}

Thank you.

[samsonasik]

Could you add additional fixture with parentheses in ternary->else which ternary->else itself a ternary with parentheses like in PR:

• [Php70] Keep right parentheses on TernaryToNullCoalescingRector #7889

Then apply the if patch if needed.

Thank you.

[TomasVotruba]

@samsonasik Just looking into this PR.
Could you add the fixture you talk about yourself? I'd like to merge it today/tomorrow

[samsonasik]

@TomasVotruba I've just checked that this

$w = !is_null($c = $x) ? ($c = $x) : $d; 

cause change to

$w = $c = $x ?? $d;

so it still needs improvement, I will continue to separate PR.

still needs improvement on parentheses usage

[samsonasik]

@Soean I've cherry-picked your commits at PR:

• [Php70] Allow is_null() conversion on TernaryToNullCoalescingRector with parentheses handling #7969