Use of field keyword / Display/Set language keyword

[erichiller]

Is your feature request related to a problem? Please describe.

When I tried to use the new field keyword, the keyword was not recognized. So I tried to find either what the current language version is, and hopefully how to change it - but I couldn't find either. The only clue I could find was in CSharpKernel.cs

Describe the solution you'd like

Ideally, the language version would be able to be configured.

Describe alternatives you've considered

If setting the language version is not yet possible, at a minimum list in the docs what the current language version is (and therefore what language features are available). (please add to the docs either way).