Improved interpolated strings

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: Not Started

Split out from #2302.

Specification is here: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/improved-interpolated-strings.md

LDM Notes:

https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-03-01.md#interpolated-string-improvements
https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-03-15.md#interpolated-string-improvements
https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-03-24.md#improved-interpolated-strings
https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-08-25.md