feat(platforms): Add support for custom platform plugins

[bundyo]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• Tests for the changes are included.

What is the current behavior?

Currently there is no way to plug a custom platform for a {N} app.

What is the new behavior?

The changes allow to start Webpack-only build for custom platforms with

npx webpack --env.platform desktop --watch

And it will build the resources in /platforms/${platform}/app/

A sample platform can be https://github.com/bundyo/nativescript-platform-desktop
A sample app - https://github.com/bundyo/ns-desktop-app

Implements #1122.

[rosen-vladimirov]

test cli-run cli-templates cli-debug cli-smoke webpack-apps ngapps

[bundyo]

test cli-run cli-templates cli-debug cli-smoke webpack-apps ngapps

[bundyo]

test cli-run cli-templates cli-debug cli-smoke webpack-apps ngapps