Support new C# 11 features

[dkearns]

C# 11 features:

• [X] Static abstracts in interfaces - no changes required
• [X] Checked user-defined operators - https://github.com/nickspoons/vim-cs/pull/63
• [ ] Unsigned right-shift operator
• [ ] Relaxing shift operator
• [ ] Numeric IntPtr
• [ ] Raw string literals - https://github.com/nickspoons/vim-cs/pull/58
• [X] Interpolated string newline - no changes required
• [X] UTF-8 string literals - https://github.com/nickspoons/vim-cs/pull/62
• [ ] Pattern match span
• [ ] List patterns
• [ ] Required members
• [ ] Auto-default struct
• [ ] Low-level struct improvements
• [ ] Extended nameof scope
• [ ] File local types
• [ ] Generic attributes