Scoped @import (plugin) functions loading

- Limited @import (plugin) support to add/addMultiple of functions
- Altered @import (plugin) loading to support browser
- Support proper closure scoping of @import (plugin) loaded functions

test/css/import-plugin.css

@@ -1,3 +1,3 @@
 .test {
-  result: OK;
+  result: PASS;
 }