feat: add --platforms flag support to flutter_app template

[marcossevilla]

Description

Add the ability to use command-line flags to specify which platform directories should be generated when running very_good create flutter_app.

Rather than creating all platform directories by default, developers want granular control over which platforms (iOS, Android, Web, macOS, Linux, Windows) are generated as part of the initial project setup.

Requirements

• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

No response

[unicoderbot]

Great, I will work on a PR and push it soon.

• Preparing environment
  • Cloning repository
  • Setting up dev environment
• Working on changes
• Reviewing changes
• Opening pull request

I've opened a pull request: #1620