Formatter must unregister when being disabled

[jrieken]

A formatter that is registered with VSCode must be unregistered when the formatter is being disabled. Otherwise VSCode still thinks a formatter is available, enabling formatting actions and contributing to the problem of conflicting formatters. See microsoft/vscode#70314 for more details on the topic and feel free to reach out to us for further questions.

Thanks & HappyCoding

[NikolasKomonen]

@JPinkney Are you working on this issue already? I'll work on it if you haven't started yet.

[JPinkney]

@NikolasKomonen I haven't started it, you'll just have to make sure its consistent with the changes in #126 (which I'll probably fixup tomorrow night)