[CodeQuality] Add InitVariableDefaultBeforeNullCoalesceReturnRector

[TomasVotruba]

Adds InitVariableDefaultBeforeNullCoalesceReturnRector (CodeQuality).

When a variable is only ever filled as an array dimension inside a loop and then returned via ?? <default> to guard the case where the variable was never initialized, this rule moves the default into an explicit init before the first write and drops the null coalesce.

 function run(array $keys): array
 {
+    $result = [];
     foreach ($keys as $key) {
         $result[$key] = $key;
     }

-    return $result ?? [];
+    return $result;
 }

Safe gates

• return expr is Coalesce, left side is a plain variable, right side is a literal default ([], '', 0, null, ...)
• the variable is never assigned directly ($x = ...) in the scope (else the ?? guards a real value, skip)
• the variable is written at least once as an array dimension ($x[...] = ...)
• the init is inserted directly before the first statement that contains the array-dim write