Always open React Native packager with iTerm

On macOS, .command files open with Terminal.app by default.

To always open the React Native packager with iTerm, associate .command files with iTerm:

• Open Finder
• Find any file with the .command extension ($PROJECT/node_modules/react-native/scripts/launchPackager.command)
• Right-click on .command file to bring up context menu
• Choose Get Info
• Change Open with to iterm.app
• Click Change All...