Support C#11 file-local types

[pmg23]

C#11 introduced file-local types, e.g.:

file Widget { }

For such a type TypeNameFormatter's GetFormattedName with default options returns a string like:

<Widget>F17747E0EBC6415B90778A501AEC22976F1C544F0DC79EFDAF3C948D89987F740__Widget

I suggest that with default options the result should be simply Widget, corresponding to the name in C# code.

Perhaps there should be a new option to opt out of that, analagous to NoAnonymousTypes.

Note that my proposal opens the possiblility of collisions where (distinct) file-local types of the same name and namespace in different files would map to the same formatted name, even in Namespaces mode. I think that is fine - there is precedent for allowing collisions, for example in the case of anonymous types.