Fix web assets not loading in Metro for web on Windows

[EvanBacon]

Why

• fix Windows has Issues Bundling Assets on Web router#120

How

• Normalize slashes on Windows to ensure they match the replacement key.

Test Plan

• Added tests.
• Need @byCedric to validate that the following renders an image on web when using Windows:

// Set `expo.web.bundler: 'metro'`

import { Image } from 'react-native';

// Requires an image file: ./icon.png
const icon = require('./icon.png');

export default function App() {
  return (
      <Image source={icon} style={{ width: 100, height: 100 }} />
  );
}

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for expo prebuild & EAS Build (eg: updated a module plugin).

[byCedric]

Can confirm that this fixed web on Windows.

• Created a new expo-router project
• Added assets from default template
• Loaded icon as described in snippet
• Ran npx expo start --web (which didn't work)
• Ran expod start --web (where `expod is an alias to the source) - and that did work.