Completion: after CSS namespace tag, suggest mx and spark namespaces in Flex projects

```css
@namespace mx "library://ns.adobe.com/flex/mx";
@namespace s "library://ns.adobe.com/flex/spark";
```

The Royale compiler doesn't currently provide good positions within the MXML document for CSS, so a fix will need to be released before this can be implemented.

apache/royale-compiler@2abad7b4e2d217fc6ea3a033b7837b7ab3209805
apache/royale-compiler@6331da32d67434e836911d25e56b1649d2079121
apache/royale-compiler@bdcb50cb1a4cfda4c666acfcef78701e85a4fa2b

Start with the CSS namespace example in the issue and review the three referenced apache/royale-compiler commits. First confirm that the Royale compiler now provides usable MXML positions for CSS; done means Flex projects suggest the mx and spark namespaces after an appropriate CSS namespace tag.

Written by the indexing model from the issue text.