add support LineStrip for PrimitiveType

[SivanChan]

I have a project requirement where I need to extend polylines into polygons. A typical approach is to use line segment strips to generate polygons in a geometry shader. However, it was discovered that nvrhi's PrimitiveType does not support LineStrip.
This is a very useful feature and I hope it can be added.

[GaelCathelin]

Hello SivanChan,

I posted several days ago an issue (#61) where I link to several small fixes and features additions, including adding the line strip primitive. I only did the Vulkan backend though but this feature is really trivial to expose.

[apanteleev]

#64 is merged, closing this issue.
Thanks @GaelCathelin !