feat: expose `keyboardWindow` (#1523)

## 📜 Description

Expose `UIWindow.keyboardWindow` as `UIWindow?` property.

## 💡 Motivation and Context

These are common changes necessary for:
- new `KeyboardEffects` component;
- re-implementation of iOS internals.

I wouldn't like to duplicate them in every branch so decided to create a
common branch and merge that into main.

Since this functionality is not used yet in the package it's safe to
release it even in patch version (but yes, this is a dead code at the
moment).

## 📢 Changelog

<!-- High level overview of important changes -->
<!-- For example: fixed status bar manipulation; added new types
declarations; -->
<!-- If your changes don't affect one of platform/language below - then
remove this platform/language -->

### iOS

- expose `UIWindow.keyboardWindow`;

## 🤔 How Has This Been Tested?

Tested via e2e tests by this PR.

## 📸 Screenshots (if appropriate):

N/A

## 📝 Checklist

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