feat: New Core/ library (#1975)

Moves everything Camera related into `core/` / `Core/` so that it is better encapsulated from React Native.

Benefits:

1. Code is much better organized. Should be easier for collaborators now, and cleaner codebase for me.
2. Locking is fully atomically as you can now only configure the session through a lock/Mutex which is batch-overridable
    * On iOS, this makes Camera startup time **MUCH** faster, I measured speedups from **1.5 seconds** to only **240 milliseconds** since we only lock/commit once! 🚀 
    * On Android, this fixes a few out-of-sync/concurrency issues like "Capture Request contains unconfigured Input/Output Surface!" since it is now a single lock-operation! 💪 
3. It is easier to integrate VisionCamera outside of React Native (e.g. Native iOS Apps, NativeScript, Flutter, etc)

With this PR, VisionCamera V3 is up to **7x** faster than V2

package/ios/Core/CameraSession+Focus.swift

@@ -0,0 +1,82 @@
+//
+//  CameraSession+Focus.swift
+//  VisionCamera
+//
+//  Created by Marc Rousavy on 11.10.23.
+//  Copyright © 2023 mrousavy. All rights reserved.
+//
+
+import AVFoundation
+import Foundation
+
+extension CameraSession {
+  /**
+   Focuses the Camera to the specified point. The point must be in the Camera coordinate system, so {0...1} on both axis.
+   */
+  func focus(point: CGPoint) throws {
+    guard let device = videoDeviceInput?.device else {
+      throw CameraError.session(SessionError.cameraNotReady)
+    }
+    if !device.isFocusPointOfInterestSupported {
+      throw CameraError.device(DeviceError.focusNotSupported)
+    }
+
+    do {
+      try device.lockForConfiguration()
+      defer {
+        device.unlockForConfiguration()
+      }
+
+      // Set Focus
+      if device.isFocusPointOfInterestSupported {
+        device.focusPointOfInterest = point
+        device.focusMode = .autoFocus
+      }
+
+      // Set Exposure
+      if device.isExposurePointOfInterestSupported {
+        device.exposurePointOfInterest = point
+        device.exposureMode = .autoExpose
+      }
+
+      // Remove any existing listeners
+      NotificationCenter.default.removeObserver(self,
+                                                name: NSNotification.Name.AVCaptureDeviceSubjectAreaDidChange,
+                                                object: nil)
+
+      // Listen for focus completion
+      device.isSubjectAreaChangeMonitoringEnabled = true
+      NotificationCenter.default.addObserver(self,
+                                             selector: #selector(subjectAreaDidChange),
+                                             name: NSNotification.Name.AVCaptureDeviceSubjectAreaDidChange,
+                                             object: nil)
+    } catch {
+      throw CameraError.device(DeviceError.configureError)
+    }
+  }
+
+  @objc
+  func subjectAreaDidChange(notification _: NSNotification) {
+    guard let device = videoDeviceInput?.device else {
+      return
+    }
+
+    try? device.lockForConfiguration()
+    defer {
+      device.unlockForConfiguration()
+    }
+
+    // Reset Focus to continuous/auto
+    if device.isFocusPointOfInterestSupported {
+      device.focusMode = .continuousAutoFocus
+    }
+
+    // Reset Exposure to continuous/auto
+    if device.isExposurePointOfInterestSupported {
+      device.exposureMode = .continuousAutoExposure
+    }
+
+    // Disable listeners
+    device.isSubjectAreaChangeMonitoringEnabled = false
+  }
+}