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

[hettiger]

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.