[CLI] Take input for npm package name in all CLI generators with a default behaviour

## Feature proposal
Allow users to enter a npm package name for app and extensions. While they can still go change in package.json, there remains a doubt in user's mind if all required changes are done to change the package name. We can show a default suggestion based on the application or extension name after checking for valid package name. For a custom entered name, we still need to check for name validity.

## Current Behavior

Currently npm package name is auto generated based on the project name. This holds for both extensions and app.

## Expected Behavior

An extra option should come in CLI asking for a package name where users can choose to provide their scoped package names or any other name they want. Generator checks for the validity and shows a message if the package name is invalid. If everything is fine, it uses the provided name for npm package name.

Start by locating the CLI generator code that currently derives package names, then compare the app and extension generator flows. Done means both flows prompt for an optional package name, suggest a valid default, validate custom input, and use the accepted name in package.json.

Written by the indexing model from the issue text.