No placeholder def for `List` type

Unlike all the other built-in types, List has no placeholder definition. I would expect that it would be handled similar to Int, String, etc.:

type Int = Int -- NOTE: The compiler provides the real implementation.

Without a placeholder...

• IDE/editor go-to-definition has nowhere to navigate to when a user tries to see what List is.
• IDEs/editors need to special-case List differently from all other types
  • this entire block of code could be removed
  • these properties could be removed

Previously addressed here: https://github.com/elm/core/issues/965

Likely needs some changes in the compiler for this to work. If so, it would come as part of a compiler release.