Test plan for "newlines in interpolations"

Championed issue: https://github.com/dotnet/csharplang/issues/4935
Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/new-line-in-interpolation.md

## Compiler
- [x] LangVer


## Productivity
- [ ] UpgradeProject needs to handle new error code