lang: core: local: Add a new vardir function

This gives us a function to return a created vardir folder. It is not
locally namespaced, and a future function will have to namespace one to
each scope.

examples/lang/vardir.mcl

@@ -0,0 +1,6 @@
+import "fmt"
+import "local"
+
+print "debug" {
+	msg => local.vardir("foo1/"),
+}