Use npm's semver rules for plugin's nativescript key

Problem with plugins verification with runtimes

Currently each NativeScript plugin must have nativescript key in it's package.json and define the minimum required version of each supported runtime. The structure is :

"name": "my-plugin",
"version": "1.0.0",
"nativescript": { 
    "platforms": {
        "ios": "1.5.0",
        "android": "1.5.0",
    }
}

When tns plugin add my-plugin is executed, CLI checks the values against current runtimes versions and informs the user if a plugin cannot be used with current version.
However CLI uses the specified version in the plugin as minimum required. So for example in case the project uses tns-android 1.6.0, 1.6.2 or even 2.0.0 for example, it can use my-plugin.
This makes it really hard to control the versions against which the plugin is tested and verified by the plugin author.

Suggested solution

Do not reinvent the wheel!
Use semver rules for the nativescript key in package.json, for example:

"name": "my-plugin",
"version": "1.0.0",
"nativescript": { 
    "platforms": {
        "ios": "^1.5.0",
        "android": "~1.5.0",
    }
}

This way in case the project targets android 1.5.2, the plugin can be installed, but in case it targets 1.6.0, the plugin will not be available. More information for the version ranges:
https://docs.npmjs.com/getting-started/semantic-versioning#semver-for-consumers

Breaking change

In case we implement this idea, all plugins, which currently has nativescript key inside their package.json, will not work, as the values: "1.5.0" will mean - can work with this version only.

Inizia con il flusso di verifica di tns plugin add e la sezione nativescript dei file package.json dei plugin; verifica come vengono attualmente confrontate le versioni del runtime. Usa gli intervalli semver documentati da npm, in modo che i valori esatti, con caret e con tilde producano il comportamento di compatibilità proposto, e verifica con dei test sia le versioni del runtime supportate sia quelle rifiutate.