Switch to browser WebExtension API namespace

[TheUnlocked]

Followup to #21

Firefox supports the chrome namespace, but Chrome does not. That said, the browser namespace is more likely to be supported in general (e.g. Edge uses browser) so I might want to switch over to that using the polyfill at some point. This is not urgent at all and hopefully I'll never have to do this because Chrome will start supporting the browser namespace, but we'll see.