C# 11.0 feature support

[damieng]

Issue to track the various parts of C# 11 support - currently in preview so subject to change before then.

Some C# 11 syntax works already as we are not as strict as Roslyn, specifically:

• [x] Generic attributes
• [x] Static abstract interface methods
• [x] Newlines in interpolated strings

Some remaining support is required for:

• [x] List patterns
• [x] Slice patterns
• [ ] Raw string literals #222