Clean up abstract classes that only contain static methods

[kenzieschmoll]

Instead of

abstract class Foo {
  // only static members
}

We can use

extension Foo on Never {
  // only static members
}

Extensions are a bit more restricted (can't be implemented/extended etc), and don't create a real type (they disappear at runtime entirely) so they are technically cheaper. This will also allow us to remove several lint ignores of avoid_classes_with_only_static_members.