array_unshift: Add fast optimized case for packed arrays (#20353)

Packed arrays are likely common in this case, as with array_shift which
already has a similar optimization.

For the following benchmark:
```php
<?php

for ($i = 0; $i < 10000000; $i++) {
    $a = [0, 1, 2, 3, 4, 5];
    array_unshift($a, -3, -2, -1);
}
```

On an i7-4790:
```
Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     753.8 ms ±  23.8 ms    [User: 749.8 ms, System: 2.1 ms]
  Range (min … max):   734.3 ms … 818.6 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     972.5 ms ±   5.0 ms    [User: 968.8 ms, System: 1.4 ms]
  Range (min … max):   967.8 ms … 984.3 ms    10 runs

Summary
  ./sapi/cli/php x.php  ran
    1.29 ± 0.04 times faster than ./sapi/cli/php_old x.php
```

UPGRADING

@@ -125,4 +125,5 @@ PHP 8.6 UPGRADE NOTES
 
 - Standard:
   . Improved performance of array_fill_keys().
+  . Improved performance of array_unshift().
   . Improved performance of array_walk().