Add type aliases from disappeared js._ types to their Scala types.

This encodes in the type system itself that the default type
to use to represent a JavaScript number (resp. boolean, string, and
undefined) is scala.Double (resp. scala.Boolean, java.lang.String,
and scala.Unit).

It will also allow most codebases to upgrade smoothly to actual
primitive JS types having been hidden away in js.prim._.