emit #nullable enable pragma in generated C# code

[robinei]

Right now the bond-generated C# code allows null references for all reference types.

This is because all generated code is excluded from nullability analysis and references are implicitly treated as nullable (<Nullable>enable</Nullable> in the .csproj does not apply to generated code). The reason for this is to not break code generators.

The intention is that code generators should opt in using the #nullable enable pragma at the top of emitted files. The bond compiler should do this, otherwise non-nullability does not apply.

This should be enabled with a command line option to the bond compiler. And possibly picked up from a property in the project settings (for the project that has and compiles the bondfile).