feat: add --platforms flag support to flutter_app template

[very-good-code-bot]

Summary

Adds support for the --platforms command-line flag to the flutter_app template, allowing developers to specify which platform directories (Android, iOS, macOS, Web, Windows) should be generated during project creation.

Changes

• Added --platforms multi-option flag to CreateFlutterApp command
• Defaults to all platforms when not specified: android, ios, macos, web, windows
• Allows granular control via CLI: --platforms=android,ios
• Updated help text with per-platform descriptions
• Passes selected platforms to template variables
• Added comprehensive test coverage including:
  • Default behavior (all platforms)
  • Custom platform selection
  • Updated usage documentation

Testing

• New test case for custom platform selection
• Updated existing tests to include platform variables
• Usage documentation reflects new flag and options

Closes #1617

[marcossevilla]

converting to draft until flutter_app template is bumped to the newer version that supports --platforms