Don't exclude resources/generator-config.json from package archives

[DaveLegg]

#903 marked resources/generator-config.json export-ignore in .gitattributes alongside genuinely dev-only files (tests/, resources/models/, resources/apis.json, resources/metadata/). Unlike those, this file is read at runtime:

// src/Generator/Package.php
public static function version(): string
{
    $config = json_decode(file_get_contents(GENERATOR_CONFIG_FILE), true);
    return $config['version'];
}

and SellingPartnerApi::defaultUserAgent() calls Package::version() on every single request to build the User-Agent header:

$version = Package::version();
$this->userAgent = "jlevers/selling-partner-api/v$version/php";

Since v7.4.9, anyone installing via Composer gets a package archive missing this file, so every SP-API request fatals:

file_get_contents(.../resources/generator-config.json): Failed to open stream: No such file or directory
Thrown in .../src/Generator/Package.php on line 14

I checked whether the other newly-excluded resource files (apis.json, metadata/**, models/**) are also read outside src/Generator/ at runtime - they aren't (only resources/feeds.json and resources/reports.json are read at runtime, via DownloadsDocument/UploadsDocument/RestrictedReport, and neither of those is in the export-ignore list), so this is the one file that needs to come back.

Verified with git archive HEAD --format=zip that the file is included in the resulting archive after this change.

Reports the same underlying issue as #906.

[jlevers]

Jesus, thanks for the catch. Merging and cutting a new release now