Checker for the available disk space

For my project, I've implemented checker for the available disk space. Configuration looks like this:
```php
\App\Libraries\SelfDiagnosis\Checks\AvailableDiskSpace::class => [
'paths' => [
'/' => '100G', // At least 100G should be available for the path "/"
'/var/www' => '5G',
],
],
```
If project maintainers will be interested, I can send PR

Start by reading the self-diagnosis checks around App\Libraries\SelfDiagnosis\Checks\AvailableDiskSpace and trace how checks are configured and reported. The work is complete when configured paths are checked against their available-space thresholds and the result integrates with the existing self-diagnosis behavior; no test file is mentioned in the issue.

Written by the indexing model from the issue text.