Add missing property declaration to fix PHP 8.2 deprecation warning

[villfa]

This PR aims to fix #231.

See PHP documentation for more context.

[villfa]

Hello @tihomiro,
I saw you made a new release a few weeks ago — great to see the project moving forward!
Would you mind taking a look at this PR and considering it for a future release? 🙏
Every time I run composer update, my console gets cluttered with the logs below 😞

local.WARNING: ErrorException: Creation of dynamic property Orangehill\Iseed\Iseed::$files is deprecated in vendor/orangehill/iseed/src/Orangehill/Iseed/Iseed.php:43
Stack trace:
#0 vendor/laravel/framework/src/Illuminate/Support/helpers.php(445): Illuminate\Foundation\Bootstrap\HandleExceptions->Illuminate\Foundation\Bootstrap\{closure}(Object(Illuminate\Log\Logger))
#1 vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(102): with(Object(Illuminate\Log\Logger), Object(Closure))
#2 vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(71): Illuminate\Foundation\Bootstrap\HandleExceptions->handleDeprecationError('Creation of dyn...', '/Users/fabien.v...', 43, 8192)
#3 vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(255): Illuminate\Foundation\Bootstrap\HandleExceptions->handleError(8192, 'Creation of dyn...', '/Users/fabien.v...', 43)
#4 vendor/orangehill/iseed/src/Orangehill/Iseed/Iseed.php(43): Illuminate\Foundation\Bootstrap\HandleExceptions->Illuminate\Foundation\Bootstrap\{closure}(8192, 'Creation of dyn...', '/Users/fabien.v...', 43)
#5 vendor/orangehill/iseed/src/Orangehill/Iseed/IseedServiceProvider.php(36): Orangehill\Iseed\Iseed->__construct()
#6 vendor/laravel/framework/src/Illuminate/Container/Container.php(908): Orangehill\Iseed\IseedServiceProvider->Orangehill\Iseed\{closure}(Object(App\Extensions\Illuminate\Foundation\Application), Array)
#7 vendor/laravel/framework/src/Illuminate/Container/Container.php(795): Illuminate\Container\Container->build(Object(Closure))
#8 vendor/laravel/framework/src/Illuminate/Foundation/Application.php(986): Illuminate\Container\Container->resolve('iseed', Array, true)
#9 vendor/laravel/framework/src/Illuminate/Container/Container.php(731): Illuminate\Foundation\Application->resolve('iseed', Array)
#10 vendor/laravel/framework/src/Illuminate/Foundation/Application.php(971): Illuminate\Container\Container->make('iseed', Array)
#11 vendor/laravel/framework/src/Illuminate/Container/Container.php(1454): Illuminate\Foundation\Application->make('iseed')
#12 vendor/laravel/framework/src/Illuminate/Support/Facades/Facade.php(237): Illuminate\Container\Container->offsetGet('iseed')
#13 vendor/laravel/framework/src/Illuminate/Support/Facades/Facade.php(208): Illuminate\Support\Facades\Facade::resolveFacadeInstance('iseed')
#14 vendor/barryvdh/laravel-ide-helper/src/Alias.php(286): Illuminate\Support\Facades\Facade::getFacadeRoot()
#15 vendor/barryvdh/laravel-ide-helper/src/Alias.php(68): Barryvdh\LaravelIdeHelper\Alias->detectRoot()
#16 vendor/barryvdh/laravel-ide-helper/src/Generator.php(168): Barryvdh\LaravelIdeHelper\Alias->__construct(Object(Illuminate\Config\Repository), 'Iseed', '\\Orangehill\\Ise...', Array, Array)
#17 vendor/barryvdh/laravel-ide-helper/src/Generator.php(234): Barryvdh\LaravelIdeHelper\Generator->getValidAliases()
#18 vendor/barryvdh/laravel-ide-helper/src/Generator.php(81): Barryvdh\LaravelIdeHelper\Generator->getAliasesByExtendsNamespace()
#19 vendor/barryvdh/laravel-ide-helper/src/Console/GeneratorCommand.php(116): Barryvdh\LaravelIdeHelper\Generator->generate()
#20 vendor/laravel/framework/src/Illuminate/Container/BoundMethod.php(36): Barryvdh\LaravelIdeHelper\Console\GeneratorCommand->handle()
#21 vendor/laravel/framework/src/Illuminate/Container/Util.php(41): Illuminate\Container\BoundMethod::Illuminate\Container\{closure}()
#22 vendor/laravel/framework/src/Illuminate/Container/BoundMethod.php(93): Illuminate\Container\Util::unwrapIfClosure(Object(Closure))
#23 vendor/laravel/framework/src/Illuminate/Container/BoundMethod.php(35): Illuminate\Container\BoundMethod::callBoundMethod(Object(App\Extensions\Illuminate\Foundation\Application), Array, Object(Closure))
#24 vendor/laravel/framework/src/Illuminate/Container/Container.php(662): Illuminate\Container\BoundMethod::call(Object(App\Extensions\Illuminate\Foundation\Application), Array, Array, NULL)
#25 vendor/laravel/framework/src/Illuminate/Console/Command.php(211): Illuminate\Container\Container->call(Array)
#26 vendor/symfony/console/Command/Command.php(326): Illuminate\Console\Command->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Illuminate\Console\OutputStyle))
#27 vendor/laravel/framework/src/Illuminate/Console/Command.php(180): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Illuminate\Console\OutputStyle))
#28 vendor/symfony/console/Application.php(1096): Illuminate\Console\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#29 vendor/symfony/console/Application.php(324): Symfony\Component\Console\Application->doRunCommand(Object(Barryvdh\LaravelIdeHelper\Console\GeneratorCommand), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#30 vendor/symfony/console/Application.php(175): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#31 vendor/laravel/framework/src/Illuminate/Foundation/Console/Kernel.php(201): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#32 artisan(35): Illuminate\Foundation\Console\Kernel->handle(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#33 {main}  

[mariopro]

The PR solves the deprecation warning.