fix iOS 18 bug where onSpeechEnd is never called

[AndersonVanzo]

Refers to #517

[jamsch]

Just a few notes on this PR (as I work on a similar library):

• There's a bit of functionality change here to how would previously have worked on iOS. I'm pretty sure on the current version of react-native-voice for iOS, speech recognition runs continuously. On Android it stops once you've paused for a second or two and processes a final result. This PR brings the functionality similar to Android.
• So the part where you have transcript = [@" " stringByAppendingString:transcription.formattedString]; is unnecessary as this is actually meant for continuous speech transcription where multiple final segments occur in a speech recognition task.
• If this library does implement continous recognition, you'll also need to reset the _hasSeenFinalResult variable back to it's initial state once starting speech recognition, otherwise you'll see an extra prepended space on your second and subsequent speech recognition attempts, such as the following:

onSpeechPartialResults "this"
onSpeechPartialResults "this is"
onSpeechPartialResults "this is a"
onSpeechPartialResults "this is a demo"
onSpeechPartialResults "this is a demo"
onSpeechResults        " this is a demo"