[12.x] Add Arr::mapKeys() method

[david-valdivia]

This PR adds an Arr::mapKeys() method to transform array keys while preserving values.

Common use case: transforming API response keys from snake_case to camelCase.

// Transform keys
$data = ['first_name' => 'Taylor', 'last_name' => 'Otwell'];
$result = Arr::mapKeys($data, fn($key) => Str::camel($key));
// ['firstName' => 'Taylor', 'lastName' => 'Otwell']

// Prefix keys (My favorite)
$config = ['host' => 'localhost', 'port' => 3306];
$result = Arr::mapKeys($config, fn($key) => "db_{$key}");
// ['db_host' => 'localhost', 'db_port' => 3306]

The implementation follows the existing Arr::map() pattern, supporting callbacks with optional parameters (key only, or key with value).

[taylorotwell]

You can just use mapWithKeys.