backport CoreML features to macos < 14

[glaszig]

i'm on 12.7 and [MLModel predictionFromFeatures:] versions with completion handler are only available on macos 14 which is a little too cutting edge for me.

before this i'd get:

whisper-encoder-impl.m:183:17: error: no visible @interface for 'MLModel'
declares the selector 'predictionFromFeatures:completionHandler:'
    [self.model predictionFromFeatures:input completionHandler:^(id<MLFeatureProvider> prediction, NSError *predictionError) {
     ~~~~~~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[glaszig]

done.

[glaszig]

done. also squashed commits.

[glaszig]

oops. fixed.