fix: do not force cast KVO values (#359)

## 📜 Description

Do not use force casting in swift - replaced with optional casting and
guard statements.

## 💡 Motivation and Context

In sentry one of library user seen this:

```bash
Thread 0 - (TH_STATE_WAITING)
0  Trusted +0x5746c0        Swift runtime failure: Unexpectedly found nil while unwrapping an Optional value (KeyboardMovementObserver.swift:109:65)
1  Trusted +0x5705cc        KeyboardMovementObserver.observeValue(forKeyPath:of:change:context:) (<compiler-generated>)
2  Foundation +0x99e54      _NSKeyValueNotifyObserver
3  Foundation +0x99cf0      _NSKeyValueDidChange
4  Foundation +0x99b20      -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:maybeNewValuesDict:usingBlock:]
5  Foundation +0x99850      -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:]
6  Foundation +0x12cb94     __NSSetPointValueAndNotify
7  UIKitCore +0x615f4       -[UIView(Geometry) _applyISEngineLayoutValuesToBoundsOnly:]
8  UIKitCore +0x60e44       -[UIView(Geometry) _resizeWithOldSuperviewSize:]
9  CoreFoundation +0x353f0  ___NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__
```

It was very random crash, but still - it's not a good practice to use
force casting and as a prof -> it produces a crash in certain cases.

So in this PR I replaced it with more safe operators.

Closes
#93 (comment)

## 📢 Changelog

### iOS

- do not force cast KVO values

## 🤔 How Has This Been Tested?

There is no way to test whether crashes are gone or not, but I think
they should gone (in worst case we will have a new stacktrace).

## 📝 Checklist

- [x] CI successfully passed
- [x] I added new mocks and corresponding unit-tests if library API was
changed

ios/observers/FocusedInputObserver.swift

@@ -164,8 +164,7 @@ public class FocusedInputObserver: NSObject {
     change _: [NSKeyValueChangeKey: Any]?,
     context _: UnsafeMutableRawPointer?
   ) {
-    // swiftlint:disable:next force_cast
-    if keyPath == "center", object as! NSObject == currentInput! {
+    if keyPath == "center", object as? NSObject == currentInput {
       // we need to read layout in next frame, otherwise we'll get old
       // layout values
       DispatchQueue.main.async {

ios/observers/KeyboardMovementObserver.swift

@@ -105,8 +105,7 @@ public class KeyboardMovementObserver: NSObject {
     change: [NSKeyValueChangeKey: Any]?,
     context _: UnsafeMutableRawPointer?
   ) {
-    // swiftlint:disable:next force_cast
-    if keyPath == "center", object as! NSObject == _keyboardView! {
+    if keyPath == "center", object as? NSObject == _keyboardView {
       // if we are currently animating keyboard -> we need to ignore values from KVO
       if displayLink != nil {
         return
@@ -117,8 +116,10 @@ public class KeyboardMovementObserver: NSObject {
         return
       }
 
-      // swiftlint:disable:next force_cast
-      let keyboardFrameY = (change?[.newKey] as! NSValue).cgPointValue.y
+      guard let changeValue = change?[.newKey] as? NSValue else {
+        return
+      }
+      let keyboardFrameY = changeValue.cgPointValue.y
       let keyboardWindowH = keyboardView?.window?.bounds.size.height ?? 0
       let keyboardPosition = keyboardWindowH - keyboardFrameY
       let position = CGFloat.interpolate(