feat(ios): use PHPickerViewController for iOS 14+ (#937)

- Does not need any permissions for reading images
- The PHPickerViewController class is an alternative to UIImagePickerController. PHPickerViewController improves stability and reliability, and includes several benefits to developers and users, such as the following:
- Deferred image loading and recovery UI
- Reliable handling of large and complex assets, like RAW and panoramic images
- User-selectable assets that aren’t available for UIImagePickerController
- Configuration of the picker to display only Live Photos
- Availability of PHLivePhoto objects without library access
- Stricter validations against invalid inputs
- See documentation of PHPickerViewController: https://developer.apple.com/documentation/photosui/phpickerviewcontroller?language=objc
- Added tests for PHPickerViewController in `CameraTest.m`

* Documentation and formatting

- Document `takePicture` and `showCameraPicker` in `CDVCamera.m`
- A pragmas for UIImagePickerControllerDelegate methods and CLLocationManager methods
- Format some long methods declarations to multi-line instead single-line for better readability
- Remove unnecessry `dispatch_async(dispatch_get_main_queue() ...` in `takePicture` before calling `showCameraPicker`. This is already done in `showCameraPicker`.
- Source out code for permission denied alert dialog when accessing the camera or UIImagePickerController on iOS < 14 for picking images

* feat(ios): proper formatting of methods

- Use linux brace style: A brace have to be on a new line for method declarations
- Remove unnecessary whitespaces in method declrations

* doc: readme update

- Better document usage descriptions
- `NSPhotoLibraryUsageDescription` not needed for iOS 14+ when only picking images
- Improve formatting for xml, js
- sourceType `SAVEDPHOTOALBUM` is the same as `PHOTOLIBRARY` on Android and iOS 14+
- Use `PHOTOLIBRARY` as sourceType instead of `SAVEDPHOTOALBUM` in  photo picker example

* Android: Document `SAVEDPHOTOALBUM``

- Make clear that `SAVEDPHOTOALBUM` is the same like `PHOTOLIBRARY` and has only an effect on iOS < 14
- Format code when creating image chooser and document the request code parameter

www/CameraConstants.js

@@ -62,11 +62,17 @@ module.exports = {
      * @enum {number}
      */
     PictureSourceType: {
-        /** Choose image from the device's photo library (same as SAVEDPHOTOALBUM for Android) */
+        /**
+         * Choose image from the device's photo library.
+         **/
         PHOTOLIBRARY: 0,
         /** Take picture from camera */
         CAMERA: 1,
-        /** Choose image only from the device's Camera Roll album (same as PHOTOLIBRARY for Android) */
+        /**
+         * Same as PHOTOLIBRARY, when running on Android, or iOS 14+.
+         * On iOS older than 14, an image can only be chosen from the device's Camera Roll album
+         * with this setting.
+         **/
         SAVEDPHOTOALBUM: 2
     },
     /**