feat: native add command to add native source files to the project

[jcassidyav]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/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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• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

You have to know where and how to add native source files to your project.

What is the new behavior?

Provided commands

ns native add swift SomeClass
ns native add objective-c SomeOtherClass
ns native add java some.thing.SomeClass
ns native add kotlin some.thing.SomeOtherClass

[NathanWalker]

This addresses in part RFC: NativeScript/rfcs#50 ❤️ thank you!