Add support for *.hlsl and *.hlsli

[petamas]

Documentation: https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl

I did not find any direct source in the above docs saying that .hlsl is the standard extension, but Google's not giving me anything else, so it seem unambigiouos enough.

I've found this on the Stack Overflow answer on the .hlsl / .hlsli distinction though, and it matches existing practice I've seen in the wild: https://stackoverflow.com/questions/46809070/what-is-the-difference-between-hlsl-and-hlsli

[asottile]

generally header is meant for c / c++ headers and really isn't that useful of a tag so I'm not adding it to new things

[petamas]

That's fine by me, I only included it because it already existed, and to be able to potentially differentiate between the two extensions.