Merges the current flutter-desktop-embedding macOS framework into the
engine. Notable changes:
- All channel/codec related code is eliminated in favor of using the
existing iOS implementations.
- All .m files renamed to .mm for consistency with the iOS code.
- Some minor code changes to fix new warnings in Objective-C++ mode.
- License headers, basic format (e.g., clang-format changes) updated to
use repo style.
- Xcode project is not included; instead adds GN build rules to create
an integrated framework that combines what was the FDE library with
what is present in FlutterEmbedder.framework.
Other changes are left as follow-ups, including:
- Moving shared code out of ios/ into common/.
- Potentially improving sharing between iOS and macOS BUILD.gn.
- Class renaming; the FLE prefix will be eliminated, but that API
surface isn't stable yet, so that can be changed later.
