Support new C# 11 features

[dkearns]

C# 11 features:

• Static abstracts in interfaces - no changes required
• Checked user-defined operators - Highlight the checked operator modifier #63
• Unsigned right-shift operator
• Relaxing shift operator
• Numeric IntPtr
• Raw string literals - Add initial support for raw string literals #58
• Interpolated string newline - no changes required
• UTF-8 string literals - Highlight the UTF8 string literal suffix #62
• Pattern match span
• List patterns
• Required members
• Auto-default struct
• Low-level struct improvements
• Extended nameof scope
• File local types
• Generic attributes