Remove `debug` export from the library

[thymikee]

Describe the Feature

There are 2 ways to debug in RNTL at the moment.
The legacy way:

import { debug } from 'react-native-testing-library';
debug(<Component />); // renders shallow by default
debug.deep(<Component />); 

The correct way:

import { render } from 'react-native-testing-library';
const {debug} = render(<Component />); 
debug(); // renders deep by default
debug.shallow();

We'd like to get rid of "the legacy way". It was introduced first and proven to be worse API to the later-introduced "the correct way", which is easier to use – less typing and scoped to the component we're currently testing. It's confusing we have 2 ways to do the same thing, which also have swapped defaults, which is confusing.

[thymikee]

Closed via #328