[dev-nativescript] add target argument support

[rvanzon]

To make it possible to manually choose the device id. Now it's possible to use the second argument for this. It's optional, if not provided, the list of available devices will show up to choose from.

• feat: added usage information

• feat: added target argument support

• refact: replaced global platform with function arguments

• refact: moved platform argument validation out of getAvailableDevices to main function

• chore: removed fs package, as it's unused

[rvanzon]

@NathanWalker FYI