Switches the Windows embedding from the standard C API to the new proctable version, to allow for unit testing of the embedding layer separately from the embedder APIs implementation. This includes moving some engine messaging that was still in flutter_windows to the C++ engine class to better encapsulate the proc table.
This does some long-overdue refactoring of the spaghetti code that grew in the GLFW embedding to begin providing a clearer separation between the engine and the window. It is now possible to register plugins, and run the runloop, on a headless engine, which makes headless mode much more usable. This is useful in some automated testing environments.
There is more refactoring that should be done in the future, but this is a good incremental point to stop as the PR is already large, and it provides useful new functionality as-is.
Adds a task runner, and exposes API to allow application-level runloops to know when they need to next call the API to process engine events. Internally, sends null events to wake up the app runloop when new events are scheduled to ensure the wait time is updated accordingly.
Fixes#36420
