Point VSCode launch config to react-native-editor root

[hypest]

Currently, trying to start (attach) the debugger provided by the vscode-react-native extension ends up in error because the extension doesn't detect the gutenberg-mobile root as the RN app root. Error:

An error occurred while attaching debugger to the application. Seems to be that you are trying to debug from within directory that is not a React Native project root. 
 If so, please, follow these instructions: [https://github.com/microsoft/vscode-react-native#customization]() (error code 604) (error code 1410)

This PR updates the launch configuration to point to the react-native-editor package root, making the extension happy.

To test:

0. In a terminal, start Metro (npm ci followed by npm run start:reset), and possibly the demo app as well (npm run core android)
1. Open VSCode from the root of the gutenberg-mobile checkout
2. Git the F5 key or use the "Run > Start Debugging" menu action to attach the debugger.
3. No error should be presented by VSCode, and a Established a connection with the Proxy (Packager) to the React Native application message should be visible in the "DEBUG CONSOLE" pane in VSCode
4. If you want to test further, place a breakpoint in

   gutenberg-mobile/src/index.js

   Line 28 in 9e667f9

   require( './setup' ).default();

5. In the demo app, open the debug menu and hit "Debug"
6. Some moments later, debugger should break at the line above.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[SiobhyB]

Tested and confirmed I'm able to use the debugger with this change 🚀 🚢