A proxy for a ScalaJS "scope" field that loads scripts lazily
E.g., ScalaJS.c, which is a scope with the Scala.js classes, can be
turned to a LazyScalaJSScope. Upon first access to a field of ScalaJS.c,
say ScalaJS.c.scala_Option, the script defining that particular
field will be loaded.
This is possible because the relative path to the script can be derived
from the name of the property being accessed.
It is immensely useful, because it allows to load lazily only the scripts
that are actually needed.