examples : add whisper.swiftui demo app (#308)

* Add SwiftUI demo project.

* Add -DGGML_USE_ACCELERATE

examples/whisper.swiftui/whisper.cpp.swift/LibWhisper.swift

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+import Foundation
+
+enum WhisperError: Error {
+    case couldNotInitializeContext
+}
+
+// Meet Whisper C++ constraint: Don't access from more than one thread at a time.
+actor WhisperContext {
+    private var context: OpaquePointer
+    
+    init(context: OpaquePointer) {
+        self.context = context
+    }
+    
+    deinit {
+        whisper_free(context)
+    }
+    
+    func fullTranscribe(samples: [Float]) {
+        // Leave 2 processors free (i.e. the high-efficiency cores).
+        let maxThreads = max(1, min(8, cpuCount() - 2))
+        print("Selecting \(maxThreads) threads")
+        var params = whisper_full_default_params(WHISPER_SAMPLING_GREEDY)
+        "en".withCString { en in
+            // Adapted from whisper.objc
+            params.print_realtime = true
+            params.print_progress = false
+            params.print_timestamps = true
+            params.print_special = false
+            params.translate = false
+            params.language = en
+            params.n_threads = Int32(maxThreads)
+            params.offset_ms = 0
+            params.no_context = true
+            params.single_segment = false
+            
+            whisper_reset_timings(context)
+            print("About to run whisper_full")
+            samples.withUnsafeBufferPointer { samples in
+                if (whisper_full(context, params, samples.baseAddress, Int32(samples.count)) != 0) {
+                    print("Failed to run the model")
+                } else {
+                    whisper_print_timings(context)
+                }
+            }
+        }
+    }
+    
+    func getTranscription() -> String {
+        var transcription = ""
+        for i in 0..<whisper_full_n_segments(context) {
+            transcription += String.init(cString: whisper_full_get_segment_text(context, i))
+        }
+        return transcription
+    }
+    
+    static func createContext(path: String) throws -> WhisperContext {
+        let context = whisper_init(path)
+        if let context {
+            return WhisperContext(context: context)
+        } else {
+            print("Couldn't load model at \(path)")
+            throw WhisperError.couldNotInitializeContext
+        }
+    }
+}
+
+fileprivate func cpuCount() -> Int {
+    ProcessInfo.processInfo.processorCount
+}

examples/whisper.swiftui/whisper.cpp.swift/WhisperCppDemo-Bridging-Header.h

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+//
+//  Use this file to import your target's public headers that you would like to expose to Swift.
+//
+#import "whisper.h"