Decide if we want to break apart monolithic interfaces, otherwise remove empty files

[joshualitt]

Today we implement partial interfaces by just collecting everything into a single interface. This results in empty files, among other issues. We could fix this by breaking apart monolithic interfaces, and putting the partial interfaces alongside the WebIDL files that implement them. However, we intend to migrate package:web to inline classes once they are available, and inline classes don't support mixins. We could continue to use extensions for this behavior, but that means these methods will not be 'sticky', meaning users will have to import package:web everywhere they want to use these methods. On the other hand, this also makes it easier for users to override the methods in package:web.

We should decide if we want to break apart monolithic interfaces, and if so we should do that now. If not, we should remove empty files from package:web.