[WIP] Rename FlutterMacOS.framework to Flutter.framework and add xcframework

[vashworth]

Changes includes:

• Rename FlutterMacOS.framework -> Flutter.framework
• Create a new version of FlutterMacOS.framework that just imports the Flutter.framework
• Create an xcframework for Flutter.framework
• Create an xcframework for FlutterMacOS.framework

Fixes flutter/flutter#70413.

Part 1 of flutter/flutter#126016.

To test it locally you can build the engine with a command like ./flutter/tools/gn --mac --mac-cpu=arm64 --no-goma --unoptimized && ninja -C out/mac_debug_unopt_arm64

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[vashworth]

Closing for now since my priorities have shifted.