Add support for C# 10 and C# 11

[danielgreen]

The last published package (1.0.0-alpha2-00395) does not support C# 10 or C# 11 language features, such as file-scoped namespaces.

E.g. if a Razor Page's code-behind file contains a file-scoped namespace, R4MVC processes it as if it was a view-only page so only emits a limited amount of generated code.

PR raised here:
#179