Describe the solution you'd like
Show a checkbox list of available platforms when the command is executed (similar to Settings Sync: Configure... and Remote: Install Local Extensions in...):

Also it would be nice to have Flutter: Manage Platforms command which essentially will do the same thing but in an existing project.

Describe alternatives you've considered
While it's possible to set required platforms via --platforms when running flutter create in a terminal even after the project is created, it would be a bit easier and faster to set them during a project creation in vscode.