Provide a package selection dialog when installing skills

[jakemac53]

This provides not only a bit more transparency into what exactly is being installed, but also lets you pick and choose which packages skills you ultimately do install. If packages are explicitly given, the dialog is skipped.

• Adds support for multiple trailing arguments so that multiple packages can be explicitly specified on the CLI
• Adds a magic all package name that just means install for all packages.
• Require either explicit package names or dialog support when installing/removing packages.
• Updated the DialogSupport.showMultiSelect method to take a set of initially selected indices
  • We use this to select all packages by default.

Closes dart-lang/ai#457