feat: add new assertScreenshot command

[SosenWiosen]

Proposed changes

This PR implements the assertScreenshot command for visual regression testing. It's based on #2078 by @TheKohan.

Given a simple React Native App

import React from 'react';
import { Button, StyleSheet, View } from 'react-native';

export default function App() {
  const [toggle, setToggle] = React.useState(false);
  return (
    <View style={styles.container}>
      <View
        testID="dot"
        style={[styles.dot, { backgroundColor: toggle ? 'red' : 'blue' }]}
      >
        <Button title="test" onPress={() => setToggle(!toggle)} />
      </View>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    alignItems: 'center',
    marginTop: 200,
  },
  dot: {
    backgroundColor: 'red',
    padding: 30,
    borderRadius: '50%',
  },
});

And with a sample flow:

appId: com.maestrotestapp
---
- launchApp
- tapOn: "test"
- assertScreenshot:
    path: screen.png
    cropOn:
      id: dot
    thresholdPercentage: 95.3
    

Maestro will compare the screenshots:

 ║                                                                                        
 ║  > Flow: test-flow
 ║                                                                                        
 ║    ✅   Launch app "com.maestrotestapp"
 ║    ✅   Tap on "test"                                                                  
 ║    ❌   Assert screenshot matches screen.png (threshold: 95.3%) (cropped on id: dot)
 ║                                                                                        
  
Comparison error: Assert screenshot matches screen.png (threshold: 95.3%) (cropped on id: dot) - threshold not met, current: 58.970116%
                                                                                          
Screenshot comparison failed. Check the diff image at /Users/sosen/WorkProjects/Maestro/screen_diff.png to see the differences. Adjust the thresholdPercentage if the differences are acceptable.

App	Correct Screenshot	Screenshot with error and diff

Testing

Two e2e tests have been added - one which should pass which compares to an already existing screenshot, and one which should fail which compares to a modified screenshot.

Issues fixed

#1222