Add Chromium support to web extension

There is an web-extension compatible with Firefox that can be used for testing until we work on integrating the API directly into the browser. This extension uses the cloneInto function from Firefox to share data between the content and background scripts. This is a Firefox-specific function; Chromium-based browsers have another way of doing this.

Since this is meant to be temporary anyway, I will not work on this personally to focus on other parts of the service, but I would review support for Chrome if someone else implements it.