feat: warn about multiple podfiles only if provided platform is unsupported

[okwasniewski]

Summary:

This PR warns about multiple podfiles only if user is using an unsupported platform directory (for example: native-ios-folder for ios root).

It also provides a useful error message when user overrides the sourceDir for given platform. For example:

For this react-native.config.js:

module.exports = {
  project: {
    visionos: {
      sourceDir: './not-exist',
    },
  },
};

We show this error:

Test Plan:

Init multi-platform project and check if the warning is there

Checklist

• Documentation is up to date to reflect these changes.
• Follows commit message convention described in CONTRIBUTING.md