Delete plugin translation files when plugin is uninstalled

[agrullon95]

Summary

Currently, plugin translation files are not deleted from /wp-content/languages/plugins when a plugin is uninstalled using wp-cli.
With these changes, file translation files will also be deleted when a plugin is uninstalled.

Testing instructions

1. Install a plugin using the following command: wp plugin install [PLUGIN-NAME]
2. Install translation files for the plugin you installed: wp language plugin install [PLUGIN-NAME] [language]

Example:

wp plugin install duplicator.
wp language plugin install duplicator fr_FR.

Access /wp-content/languages/plugins and verify you see the language files for the plugin you installed.

duplicator-fr_FR.mo
duplicator-fr_FR.po

3. Uninstall the plugin using wp plugin uninstall [PLUGIN-NAME].
4. Verify the language files no longer exist in the plugins folder.

Fixes #337

[danielbachhuber]

Thanks for your work on this, @agrullon95 !

I made one tweak to your conditional so uninstall_plugin( $plugin->file ); is always called. There was a failure otherwise.

I also added a feature test, if you want to take a look at that.