Allow building a Custom Preview App and using it with `tns preview`

Is your feature request related to a problem? Please describe.

NativeScript Playground is an amazing developer experience for some really quick tests. Sometimes I test something in the Playgrounds just because it's a great, fast and reliable sandbox. There's not always the intention to share the Playground.

From time to time I want to try something in a Playground that is not possible. That's usually because some native library is missing. I understand the reason for that. Anyways this feature request is all about a solution to this problem. A solution that allows fast, reliable testing over the air. No cables required. A solution however that is not as easy to share.

See: https://www.nativescript.org/blog/join-the-nativescript-5.0-launch-webinar#comment-4151225217

Describe the solution you'd like

Please consider adding a --custom-preview CLI param so we can:

1. Add native libraries to our own custom preview app
2. Run tns preview --hmr --custom-preview
3. Scan the QR code with a Phone and run the app

This would give us a very fast, flexible dev experience. No cables needed.

Maybe something like tns deploy ios --custom-preview could build and deploy a custom preview app with the dependencies of our package.json file.

One thing that is really important to consider when reviewing this request:

A user would be able to build one custom preview app with a whole lot of plugins and reuse that for many different codebases to quickly fiddle around. The idea is not to run tns deploy ios --custom-preview on a daily basis. This is for situations where you might want to fiddle around with a NativeScript next version or something along those lines.

tns preview --hmr のフローから始め、tns deploy ios と比較しながら、package.json の依存関係、カスタムネイティブライブラリ、QR コードによる配信、preview アプリの再利用をどのように組み合わせられるかに焦点を当てます。ユーザーがネイティブ依存関係を含むカスタム preview アプリをビルドし、それを複数のプロジェクトで再利用して、ケーブルなしで無線経由で起動できれば完了です。