Adds a new header, flutter_macros.h which includes a FLUTTER_DEPRECATED
macro that can be used to mark deprecated API as such, with a
hopefully-informative message, ideally describing the expected removal
version and any migration tips.

This will need to be #included in flutter_windows.h and flutter_linux.h,
but prior to doing so, we'll need to update the engine recipe to bundle
the new header, here:
https://flutter.googlesource.com/recipes/+/refs/heads/main/recipes/engine/engine.py#1457

No tests since this adds a compiler macro that will be used for future
C/C++ API deprecation once the above recipe change has landed;
specifically: FlutterDesktopEngineProcessMessages.

Related: flutter/flutter#93537

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.

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