Proposal: field keyword in properties

(Ported from dotnet/roslyn#8364)

Specification: https://github.com/dotnet/csharplang/blob/main/proposals/field-keyword.md

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