C# Renderer with `--features just-types` doesn't include required `using`s.

[gregmarr]

If you run the C# renderer with --features just-types --array-type list then it creates List<T> properties, but doesn't add the required using System.Collections.Generic;. It also happens without the --array-type list if a Dictionary<K,V> is generated as a property.

This is due to the checks here, with FIXMEs from 6 years ago pointing out this exact bug.

quicktype/packages/quicktype-core/src/language/CSharp.ts

Line 851 in 916cd94

 // FIXME: We need System.Collections.Generic whenever we have maps or use List. 

quicktype/packages/quicktype-core/src/language/CSharp.ts

Line 1579 in 916cd94

 // FIXME: We need System.Collections.Generic whenever we have maps or use List.