Laravel Installer: Fix --github option and add --public flag

pr-description.md

Fixed

• The --github option uses Symfony Console's VALUE_OPTIONAL mode, which silently consumes the next positional argument as the option value when no = separator is used. Running laravel new --github my-app caused my-app to be captured as the option value rather than the project name, and the installer would prompt for the name again before building a broken gh repo create command with the project name duplicated in the flags position — e.g. gh repo create my-app --source=. --push my-app.

Changed

• The --github option now accepts an optional repository name, allowing the GitHub repository to be named differently from the project directory:

  laravel new my-app --github=my-different-repo-name
  

• A new --public flag controls the visibility of the created GitHub repository. The default remains private, matching the previous behaviour:

  laravel new my-app --github --public