Add the ability to specify authored structure's name (except runtime class)

[roxk]

So that devs don't have to fully qualified their projected types to avoid name look up ambiguity.

Context:

namespace winrt::Root::implementation
{
  struct SomeStruct : idlgen::author_struct
  {
  };
}

// MainPage.h
SomeStruct Getter(); // Would lookup implementation::SomeStruct by default, devs have to write Root::SomeStruct to disambiguate

Expected:

namespace winrt::Root::implementation
{
  struct _SomeStruct : idlgen::author_struct
  {
  };
}

// MainPage.h
SomeStruct Getter(); // Would lookup Root::SomeStruct as expected

• Implement _ prefix which idlgen would strip out