Loosen restrictions on package:js/Native type conflicts to nearly empty classes

[srujzs]

Currently, package:js has checks to make sure that the class being bound to in a @JS() annotation doesn't conflict with an @Native class. For context, #44211 pointed out issues with treating native types as interop types today.

Instead of issuing an error for any interop use, we only need to issue an error for when there are instance members within the interop class that aren't external constructors. This will allow users to declare static extension methods on the interop'd class and use native types with interop statically.

[sigmundch]

Note: we will also need to change to the subtyping rules in ddc and dart2js runtimes to allow assignments from native types to this kind of "empty" js-interop types.