Merge release in master

[rosen-vladimirov]

Include the following fixes:

Fix debug on iOS simulator with watch (#2721)

During tns debug ios, in case you make changes, the application must be restarted and the debugger must attached again. However, in many cases we kill the old lldb process and immediately try to start the new one. The childProcess.kill operation finishes, but lldb process does not die immedietely. So in some occasions, the attach of new debugger fails. This leads to multiple errors - you cannot start this application on simulator anymore, you cannot exit CLI's process with Ctrl + C, etc.
Fix this by attaching to "close" event of the processes and waiting for them to be really finish their execution.

Implement extensibility model for CLI (#2724)

Implement extensibilty of CLI that allows anyone to add easily create packages that add new functionality to NativeScript CLI. The packages are installed in a specific directory, so they are persisted through CLI's updated.
The directory where extensions are installed contains a package.json and each extension is npm package installed there.
The extensions can be mainatined in two different ways:

• navigate to the directory where extensions are installed and use npm for install/uninstall/update of packages.
• use CLI's commands to update them: tns extension install <name>, tns extension uninstall <name>, tns extension

Implement extensibilityService that executes all operations and expose it to public API. In {N} CLI the extensions are loaded in the entry point, before parsing command line arguments. This way extensions can add new commands.
In Fusion, after CLI is required as a library, the extensibilityService.loadExtensions method should be called. It returns array of Promises - one for each installed extension.

Add help for the new commands, but do not link the new commands in other commands help for the moment.

Do not start emulator when --available-devices is passed (#2736)

In case there's no devices attached and no emulators running, trying <cli name> devices <platform> --available-devices will start emulator.
In order to fix this, modify the startEmulatorIfNecessary method to skip the starting in case skipInferPlatform option is passed. This option indicates that we are not concerned of specific platform, so the method does not know which is the target platform for which to start emulator.
Add unit test for this behavior.

Install karma peer dependencies on test init (#2693)

• implicitly install karma-'s peer dependencies on test init command

• add exception handling when installing packages already present in the project

• log warning instead of throwing errors when a package's name couldn't be determined when installed using the node-package-manager service