Allow internal types to be invoked by JavaScript interop

We should be able to mark `public` methods on `internal` types as `[JSInvokable]` since most of the lib developer, like myself, don't want to expose internal types to the end user.

I know that there is the idea that `JSInvokable` is inherently `public` and that mislead the idea that the types using it should be as well. I also know that you can hide a member with `EditorBrowseableAttribute(false)`, but that is just an IDE thing and not truly protecting internal APIs from misuse.

So, please consider change the scan logic to also look into `internal` types.

Thanks!