[Yexuan Xiao (YexuanXiao)]

This PR ports wil::resume_background with a few small improvements:

• Uses operator() instead of Schedule, support using lambdas to replace cumbersome struct definitions.
• Uses the destructor of dispatcher_state_guard instead of try {} catch (...) { throw; } pattern, which help compiler generates cleaner code.

C++/WinRT 3.0 should fix this issue, as it could cause coroutines to run on the wrong thread or leak. Since almost no one checks the return value of co_await, this will hardly break existing code.

C++/WinRT already has special handling for WinUI (base_xaml_component_connector_winui.h), so unless there are plans to remove that code from C++/WinRT, support for Microsoft::UI::Dispatching::DispatcherQueue should not be rejected.

The tests are also port from wil, identical except for the namespace name.

[willdieh]

PLEASE accept this pull request - having to import WIL just to do resume_foreground is unfortunate. The advanced threading documentation for cpp/winrt still have the resume_foreground example for the more general use case of obtaining the UI thread and omit the WIL requirement. It took me a whole day to figure out that WIL was required (although it is discussed in the migration docs). Looks like this pull is ready to approve 👍