Add preliminary Slang support (#7151)

Part of
https://github.com/compiler-explorer/compiler-explorer/issues/2331 and
similar to [my GLSL
change](https://github.com/compiler-explorer/compiler-explorer/pull/6883)

[Slang](https://shader-slang.com/) is a GPU focused shading language
that has been worked on for years. Last week [The Khronos
Group](https://www.khronos.org/news/press/khronos-group-launches-slang-initiative-hosting-open-source-compiler-contributed-by-nvidia)
will now be running the project as open governance. The latest [Vulkan
SDK](https://www.lunarg.com/lunarg-releases-vulkan-sdk-1-3-296-0-for-windows-linux-macos/)
has also added a build of `Slangc` (slang compiler).

This change adds support for Slang (the language) as a front end with
`Slangc` (the compiler) as the only compiler. Slang can be used for
things like GLSL/HLSL, but that is for a future PR.

I am in contacts with people on the Slang development and plan to
support things for Slang as well as the other GPU related shading
languages

--- 

of course, screen shots as well


![image](https://github.com/user-attachments/assets/c9f6460b-b779-4ff5-b0bb-5a7eff543de1)


![image](https://github.com/user-attachments/assets/b2dbdd5a-c031-4110-ab9d-ed76b9881ad8)

types/languages.interfaces.ts

@@ -83,6 +83,7 @@ export type LanguageKey =
     | 'ruby'
     | 'rust'
     | 'scala'
+    | 'slang'
     | 'solidity'
     | 'spice'
     | 'spirv'