Renames all FLE* classes in the macOS embedding to Flutter*. With the exception
of -[FlutterDartProject engineSwitches], which is very clearly called out in the
comment, the APIs should be stable at this point, so the marker prefix is no
longer needed.

This is a breaking change for macOS embedders, but going forward breaking
changes at the source level for the macOS API should now be rare.

Some of these classes will likely merge with the iOS versions in the future (e.g.,
FlutterDartProject), but that will be an implementation detail that will not affect
clients.

Fixes flutter/flutter#31735