[Proposal]: `field` keyword in properties (VS 18.0, .NET 10)

# Proposal: `field` keyword in properties

Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-14.0/field-keyword.md
Discussion: https://github.com/dotnet/csharplang/discussions/8634

## LDM history:

- https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-03-10.md#field-keyword
- https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-04-14.md#field-keyword
- https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-01-12.md#open-questions-for-field
- https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-02-16.md#open-questions-in-field
- https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-03-02.md#open-questions-in-field
- https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-03-21.md#open-question-in-semi-auto-properties
- https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-05-02.md#field-questions
- https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-26.md#ungrouped
- https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-10-12.md#keywordness-of-field
- https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-05-15.md#field-and-value-as-contextual-keywords
- https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-06-24.md#field-questions
- https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-07-15.md#field-keyword
- https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-08-14.md#field-keyword
- https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-08-21.md#field-keyword-nullability
- https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-08-26.md#field-keyword-open-questions
- https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-18.md#nullability-in-field