[5.5] allow loading JSON translations for packages

[themsaid]

In a package service provider the developer would use:

$this->loadJSONTranslationsFrom(__DIR__.'/translations');

Translations in the main json file will override package translation, otherwise package maintainers and users can use translation lines that exist in the package translation file __('Re-run job').

[taylorotwell]

use addJsonPath instead of capitalizing JSON in method names.

[themsaid]

@taylorotwell done