Fix syntax highlighting for stylus import/require

Hi,

I have a stylus file with this syntax at the top of file
```stylus
@require "../../lib/*"
```
and the syntax highlighting will break for whole file.
Furthermore, replacing require to import will break as well.

Sorry that it's my first experience of vim syntax and just reference from scss-syntax.
Please feel free to advice me if my changes is wrong.
Thanks,
Kudo