Feature request: add an all-in-one module interface file declaring `export winrt;` that `export import`s everything else

Summary

It is quite nice to see modules support massively improved for cppwinrt; this was a silent effort that went quite unnoticed by the rest of the community! Thanks to all.

That being said, I am a little concerned at the module being split up into 'namespaces'. The general consensus by build system and compiler developers is that libraries should tend towards mega-modules, as the parsing and instantiation cost is paid once upfront. I must also point out that some of the purported benefits listed here don't quite exist in C++. There is very little (amortised) cost to having a giant module interface file (this is exactly how module std works, for instance).

I therefore propose a small addition that allows users to consume cppwinrt in one go with an interface file that export imports all the separated interfaces.

Start with docs/modules-design.md and inspect the existing separated module interfaces to determine their names and export structure. Add an all-in-one interface that re-exports them, and verify that consumers can import winrt through that single interface.