Make features/field-keyword branch available

[jnm2]

The https://github.com/dotnet/roslyn/commits/features/field-keyword branch replaces https://github.com/dotnet/roslyn/commits/features/semi-auto-props in implementing https://github.com/dotnet/csharplang/blob/main/proposals/field-keyword.md for https://github.com/dotnet/csharplang/issues/140.

Implementation has just started going into the features/field-keyword branch, e.g. https://github.com/dotnet/roslyn/pull/74641, and it would be great to test it out and share examples as it gets implemented.