Fix main thread checker warnings from AVSpeechSynthesizer

[Clstroud]

When playing speech utterances in the various places in Vocable, we now get many console messages from the main thread checker:

Thread Performance Checker: Thread running at User-interactive quality-of-service class waiting on a lower QoS thread running at Default quality-of-service class. Investigate ways to avoid priority inversions
PID: 52026, TID: 9834265
Backtrace
=================================================================
3   Foundation                          0x0000000180e0f1cc -[_NSThreadPerformInfo wait] + 40
4   Foundation                          0x0000000180e1127c -[NSObject(NSThreadPerformAdditions) performSelector:onThread:withObject:waitUntilDone:modes:] + 596
5   Foundation                          0x0000000180e114e4 -[NSObject(NSThreadPerformAdditions) performSelector:onThread:withObject:waitUntilDone:] + 104
6   TextToSpeech                        0x00000001b050406c TTSSpeechTransformTextWithLanguageAndVoiceIdentifier + 4928
7   TextToSpeech                        0x00000001b0508eec AXAVSpeechSynthesisVoiceFromTTSSpeechVoice + 12152
8   TextToSpeech                        0x00000001b04ffe84 _TTSIdentifierForVoiceInformation + 8076
9   TextToSpeech                        0x00000001b0500210 _TTSIdentifierForVoiceInformation + 8984
10  TextToSpeech                        0x00000001b04fff64 _TTSIdentifierForVoiceInformation + 8300
11  TextToSpeech                        0x00000001b050253c _TTSIdentifierForVoiceInformation + 17988
12  Vocable                             0x00000001029c3170 $s7Vocable30VoiceProfilePreviewSynthesizerC04playD0yySo017AVSpeechSynthesisB0CF + 652
13  Vocable                             0x00000001029724b4 $s7Vocable29VoiceProfilePreviewControllerC04playD0yyAA0bC4ItemVF + 124
14  Vocable                             0x0000000102898a94 $s7Vocable0A24ListContentConfigurationV16voiceProfileItem_10controller0E14SelectedActionAcA05VoicefG0V_AA0kF17PreviewControllerCyycSgtFZyycfU0_ + 116
15  Vocable                             0x00000001029863c4 $s7Vocable0A19ListCellContentViewC37handleLeadingAccessoryActionSelection33_FFC001FFF6F30FECDE42E4E22536E5DELLyyAA14GazeableButtonCF + 1948
16  Vocable                             0x00000001029864a4 $s7Vocable0A19ListCellContentViewC37handleLeadingAccessoryActionSelection33_FFC001FFF6F30FECDE42E4E22536E5DELLyyAA14GazeableButtonCFTo + 52
17  UIKitCore                           0x00000001853ecf04 -[UIApplication sendAction:to:from:forEvent:] + 96
18  UIKitCore                           0x0000000184cfab38 -[UIControl sendAction:to:forEvent:] + 108
19  UIKitCore                           0x0000000184cfae7c -[UIControl _sendActionsForEvents:withEvent:] + 268
20  UIKitCore                           0x0000000184cf78f0 -[UIButton _sendActionsForEvents:withEvent:] + 120
21  UIKitCore                           0x0000000184cfaeb8 -[UIControl _sendActionsForEvents:withEvent:] + 328
22  UIKitCore                           0x0000000184cf78f0 -[UIButton _sendActionsForEvents:withEvent:] + 120
23  UIKitCore                           0x0000000184cf9b14 -[UIControl touchesEnded:withEvent:] + 392
24  Vocable                             0x000000010291e01c $s7Vocable14GazeableButtonC12touchesEnded_4withyShySo7UITouchCG_So7UIEventCSgtF + 168
25  Vocable                             0x000000010291e1b0 $s7Vocable14GazeableButtonC12touchesEnded_4withyShySo7UITouchCG_So7UIEventCSgtFTo + 100
26  UIKitCore                           0x00000001854208e4 -[UIWindow _sendTouchesForEvent:] + 900
27  UIKitCore                           0x0000000185421e08 -[UIWindow sendEvent:] + 3104
28  Vocable                             0x0000000102901bd0 $s7Vocable14HeadGazeWindowC9sendEventyySo7UIEventCF + 2856
29  Vocable                             0x0000000102901df4 $s7Vocable14HeadGazeWindowC9sendEventyySo7UIEventCFTo + 52
30  UIKitCore                           0x00000001854017c8 -[UIApplication sendEvent:] + 576
31  UIKitCore                           0x00000001854849d8 __dispatchPreprocessedEventFromEventQueue + 1684
32  UIKitCore                           0x0000000185487824 __processEventQueue + 5524
33  UIKitCore                           0x0000000185480218 __eventFetcherSourceCallback + 156
34  CoreFoundation                      0x000000018040ee88 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 24
35  CoreFoundation                      0x000000018040edd0 __CFRunLoopDoSource0 + 172
36  CoreFoundation                      0x000000018040e540 __CFRunLoopDoSources0 + 232
37  CoreFoundation                      0x0000000180408c28 __CFRunLoopRun + 768
38  CoreFoundation                      0x0000000180408514 CFRunLoopRunSpecific + 572
39  GraphicsServices                    0x000000018ef06ae4 GSEventRunModal + 160
40  UIKitCore                           0x00000001853e8040 -[UIApplication _run] + 868
41  UIKitCore                           0x00000001853ebcc8 UIApplicationMain + 124
42  Vocable                             0x00000001029ca724 main + 64

We should consider wrapping this functionality into an actor so that we don't risk hanging the main thread. UI hangs while performing speech synthesis haven't been a huge issue to date, but perhaps we will see some minor performance gains by making this change.