fix: force server to always start in a new window on macOS

[Kamefrede]

Summary

This PR fixes an issue on macOS where the server wasn't reliably being launched in a new terminal window. The solution involves using the -na flag combination with the open command instead of just -a, which ensures that a new instance of the terminal application is always created.

As per the man entry for open:

DESCRIPTION
     The open command opens a file (or a directory or URL), just as if you had double-clicked the file's icon. If no application name is specified, the default application as determined
     via LaunchServices is used to open the specified files.

     [...]

     -n  Open a new instance of the application(s) even if one is already running.

The issue was identified in a Ghostty discussion which pointed out that some terminals work around the fact that open -a does not inherit the current environment it's being run in by explicitly injecting the environment variables, the standard and reliable way to ensure a new window is created and the environment variables are correctly set is to use the -n flag explicitly with the macOS open command.

This change ensures consistent behavior across all terminal emulators on macOS.

Test Plan

I've verified this change by:

1. Testing on macOS with multiple terminal emulators (Terminal.app, iTerm2, Ghostty)
2. Ensuring that running the server now consistently opens in a new terminal window
3. Checking that the server starts correctly in all cases

To test this yourself:

• Clone the repo and make this change
• Launch a React Native project using npx react-native start
• Verify that a new terminal window opens with the Metro server
• Try multiple times to confirm consistent behavior, especially with different terminal applications already running

Checklist

• Documentation is up to date.
• Follows commit message convention described in CONTRIBUTING.md.
• For functional changes, my test plan has linked these CLI changes into a local react-native checkout (instructions).

[szymonrybczak]

Amazing! Thank you for fixing this! I'm also Ghostty user btw :D

[szymonrybczak]

cc @huntie

[huntie]

LGTM :)