feat: make init command standalone

[tido64]

Description

Make init command standalone. The command should be able to create a new app using the already installed copy of react-native, or download one if necessary.

Platforms affected

• Android
• iOS
• macOS
• Windows

Test plan

• Verify that it uses the installed react-native: node scripts/init.js
• Verify that it downloads react-native@0.69: node scripts/init.js --version 0.69
• Verify that it determines the latest version and downloads it:

  # start outside any repository
  node /path/to/react-native-test-app/scripts/init.js
  

For each of the test, verify that the generated package.json contains the correct versions.

[kelset]

Looks great, tested all cases locally and I think we can merge it as is

A couple of things I noticed and that we can follow up on as separate PRs

• it puts as RNTA version in the package.json the one from branch, so "^0.0.1-dev" this is probably because I'm running it locally, let's verify post release that it sets to latest available on npm
• doesn't do pod install, so might be good to add as a step when it detects that the project targets iOS
• exposing Hermes flag: neither iOS nor Android side I could see an easy way to switch on/off Hermes. Maybe it's just me not looking in the right place