Add array_merge_if helper function for conditional array merging

[altas-innova-tech]

This PR adds a new array_merge_if helper function that conditionally merges arrays based on a boolean condition.

Motivation

Currently, conditional array merging requires verbose if-else blocks or ternary operators, making code less readable:

// Current approach
$config = $isProduction 
    ? array_merge($baseConfig, $prodConfig) 
    : $baseConfig;

// With array_merge_if
$config = array_merge_if($isProduction, $baseConfig, $prodConfig);

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!