Add a setting that exports all variables by default, regardless of
whether they use the `export` keyword. This includes assignments as well
as parameters.
Just does dependency analysis of variable uses, allowing variables to be
used out of order in assignments, as long as there are no circular
dependencies.
However, use of environment variable is not known to Just, so exported
variables are only exported to child scopes, to avoid ordering dependencies,
since dependency analysis cannot be done.
