This makes it much nicer to use `BrowserType` because it no longer has a template.
Technically a breaking change because of the rare edge case where someone used their own non-browser type inside the template, but I don't consider that intended behavior and think this is fine.
This makes it easier to reason about our packages.
The only difference is what each package downloads.
When the browser is not downloaded, it will fail to launch.
Each browser gets a 'download' attribute in the browser.json file.
