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-scoped.css

@@ -0,0 +1,6 @@
+.in-scope {
+  result: PASS;
+}
+.out-of-scope {
+  result: test();
+}