[12.x]: add isEmail str helper method

[KIKOmanasijev]

This PR adds a new isEmail() method to the Stringable class for quick email format validation. The method provides a convenient way to check if a string is a valid email address.

Example

Current approach requires breaking the fluent chain:

class UpdateBackupEmailAction
{
   public function execute(User $user, array $data): bool
   {
       if (isset($data['backup_email']) && !Str::isEmail($data['backup_email'])) {
           throw new InvalidArgumentException('Invalid backup email format');
       }
       
       return $user->update(['backup_email' => $data['backup_email']]);
   }
}

[shaedrich]

Is there a reason why you don't want to offer the rich validation options the email Validator rule offers?

[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!