Completion should suggest potential generic parameter name when typing

```cs
public class MyClass
{
public TReturnType M<$$>
}
```
It would be great if completion at `$$` suggested `TReturnType` as a generic parameter name since that name isn't resolved. Probably as a soft completion since user may want to have multiple parameters or generate the type after they finish typing method signature. Also maybe limit this to only when potential name starts with `T` or that's too restrictive?

No file, test, or entry point is named. Start by reproducing the C# completion scenario at the generic parameter position and trace the completion behavior for the unresolved return type name; done means a suitable potential parameter name is offered as a soft completion without preventing additional parameters or later type generation.

Written by the indexing model from the issue text.