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 dotnet/csharplang#140.

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

[jnm2]

The feature merged to main! (dotnet/roslyn#75228)