feat(Keyboard): add isVisible() method

## Feature Request

### Plugin

- Keyboard

### Description

There should be a way to determine if the keyboard is currently visible or not.

Currently the Keyboard listener `keyboardWillShow` will trigger, even if keyboard is already visible.
This happens if you got > 1 input elements and focus the next one, while keyboard is visible.
In this case `keyboardWillShow` triggers always twice (#1339).

Checking keyboard visibility would solve this issue.

### Platform(s)

- ios
- android

### Preferred Solution

extend Keyboard by method `isVisible(): Promise`

### Alternatives

Using the [cordova](https://github.com/ionic-team/cordova-plugin-ionic-keyboard) solution