[Proposal]: Raw string literal

# Raw string literal

* Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/raw-string-literal.md
* Discussion: https://github.com/dotnet/csharplang/discussions/8646

## Summary

Allow a new form of string literal that starts with a minimum of three `"""` characters (but no maximum), optionally followed by a `new_line`, the content of the string, and then ends with the same number of quotes that the literal started with. For example:

```
var xml = """

""";
```

## Design meetings