Language idea: More advanced string literals

[alexrp]

We need to come up with a design for more advanced string literals. In particular, string literals that can span multiple lines are frequently useful.

String interpolation is out of scope for this.

[alexrp]

I think we might just steal C#'s relatively new raw string literals:

• https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-11.0/raw-string-literal
• https://github.com/dotnet/roslyn/blob/main/src/Compilers/CSharp/Portable/Parser/Lexer_RawStringLiteral.cs

Not sure if verbatim string literals are useful enough to justify inclusion in some form. Need to think on that.

[alexrp]

We will basically copy C#'s approach here.

Approved for 1.0.