Add language alias to c# so it matches file extensions

[emerxom]

can we have cs for language | 'csharp' | 'c#' | 'cs'

we can import files to show the code and would be nice if the language matches the file extension so it can identify the correct language.

see https://github.com/vuejs/vitepress/issues/861 https://github.com/vuejs/vitepress/pull/875

[orta]

Sure, you're welcome to add it if it hasn't been added since this issue came out 👍🏻