Deprecation Warning Thrown on PHP8.2

## Issue

On PHP 8.2, the plugin throws the following deprecation warnings:

```php
PHP Deprecated: Creation of dynamic property Media_Explorer::$file is deprecated in /my-cool-path/wp-content/plugins/media-explorer/class.plugin.php on line 27
```

```php
PHP Deprecated: Creation of dynamic property Media_Explorer::$plugin is deprecated in /my-cool-path/wp-content/plugins/media-explorer/class.plugin.php on line 89

```

## Proposed solution

Add the props to the class so that they aren't dynamically created.

Open class.plugin.php and inspect the Media_Explorer class around lines 27 and 89, where the warnings identify dynamic properties. Define the mentioned properties on the class, then verify on PHP 8.2 that the plugin no longer emits those deprecation warnings.

Written by the indexing model from the issue text.