Fix plugin provider setup function

[rekmarks]

#19 removed the plugin metadata parameter from setupWorkerConnection. That function is actually used to setup to plugin provider connection (it ultimately calls setupUntrustedCommunication in the extension background, see here), and it needs at least the plugin name in order to do that.

This PR renames setupWorkerConnection to setupPluginProvider – which actually describes what it does – and adds a pluginName parameter.