Add slang to GitHub's linguist

[expipiplus1]

https://github.com/github-linguist/linguist

I'm pretty sure that we meet the popularity requirements by now, we'd need to open a PR which implements slang file detection (the .slang extension is unused so far so that's simple), and also link in a grammar for highlighting.

Following on from that we can switch all the code blocks from hlsl/csharp over to slang

[jkwak-work]

I couldn't figure out what linguist is. It seems like it is a tool to generate a graph like the following,

And this task is to get the graph show the percentage of Slang files in the repository, is it?

[expipiplus1]

It's also what's behind the syntax highlighting for markdown on GitHub.

[bmillsNV]

Let's schedule this for December