Add .phpstorm.meta.php to improve code completion

Folks, adding a `.phpstorm.meta.php` file with something like this below, would make users' experience with the package in PhpStorm a bit better when using calls like `->api('issue')`:
```php
\Github\Api\Issue::class,
//...
]));
}
```
Would you be interested in adding it?

Start with the proposed .phpstorm.meta.php file and inspect the \Github\Client::api(0) entry point to identify the API names that need metadata mappings. The work is complete when PhpStorm can infer the returned type for calls such as api('issue'), with the mappings covering the package's relevant API classes.

Vom Indexierungsmodell aus dem Issue-Text verfasst.