Even though the file is pure C code, it's useful to use a C++ or Objective-C++ filename in order to use FML (assertions) in the implementation.

Three implementation changes:

• Since the file is now C++, some implicit void* conversions now require an explicit cast.
• A malloc/free of a buffer has been replaced with a std::vector.
• An assert() was changed to FML_DCHECK for consistency with the rest of the embedder.

No test changes since there are no semantic changes.

Existing tests are in:
https://github.com/flutter/engine/blob/main/shell/platform/darwin/common/framework/Source/flutter_standard_codec_unittest.mm

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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