Libby's Close Tabs to the Left wot Libby

This is as basic as it gets. No localization and no further features will be added. The only changes will be optimizations and anything to make it work.

The reason it says "[ADD-ON]" is mostly so I don't forget it's an add-on and get confused why it isn't there if it breaks. I think it also helps differentiate itself from the other options, of which I never use.

This is supposed to all be one add-on, but annoyingly had to be split. Get the other parts here:
https://addons.mozilla.org/en-US/firefox/addon/libbys-close-tabs-to-the-right/
https://addons.mozilla.org/en-US/firefox/addon/libbys-close-other-tabs/