Labs: Rich-text editor: Use `UITextInputAssistantItem` on iPad

### Your use case

#### What would you like to do?

On iPad, `UITextInputAssistantItem` ([documentation](https://developer.apple.com/documentation/uikit/uitextinputassistantitem)) allows you to add extra buttons to the on-screen keyboard suggestion bar, so that they are only visible when the on-screen keyboard is.

Might look tidier and use less screen space than having formatting buttons on-screen all of the time as they are in Labs at the moment.

### Have you considered any alternatives?

_No response_

### Additional context

Unfortunately I don't think this will work for iPhone, as per the docs:

> You can add custom items to the shortcuts bar on iPad only. On iPhone, the text input system ignores the contents of the UITextInputAssistantItem object.