Deprecate support for the Legacy Workflow

[DimitarTachev]

Is your feature request related to a problem? Please describe.
In NativeScript 6.0.0, we will drop the support for building apps without --bundle (the Webpack approach will become the default and only solution) and we need to propose the recommended flow to the older projects in order to get as much feedback as possible as soon as possible.

Describe the solution you'd like
The solution is displayed on the chart below:

Additional context

• NativeScript 5.3 - Hot Module Replacement Officially Supported
• TODO: Publish and link a blog post describing the Workflow changes in 5.4 and 6.0 in more details.