Add initial support for raw string literals

[dkearns]

This is incomplete but probably does a decent job highlighting correct code.

TODO:

• Single-line and multi-line raw strings should be handled separately and more strictly.
• Invalid delimiter sequences should be highlighted as errors.
• Normal strings shouldn't obscure errors by matching in invalid raw string literal syntax.
• Should raw strings be coloured distinctly?

[nickspoons]

Thanks @dkearns. To answer your question, in my opinion no, raw strings shouldn't be highlighted differently to other strings. Despite the new semantics they are just strings at the end of the day, and people have expectations of those basic highlight colours. People who want them coloured differently can always link csRawString to something else in their personal config if they choose.

[nickspoons]

@dkearns I would quite like to merge this. How important do you feel your "TODO"s are? As you mentioned, it works well as it is

[dkearns]

Hi Nick,

Three years...sorry.

I have lost context but I had a quick look and I think it's fine to merge. At least, it appears to handle all the examples on https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/raw-string.

I'll take a look at my other outstanding PRs soon and see what else has been added to C# while I blinked.

[nickspoons]

Thanks very much @dkearns!